android: Add top-level build targets

As with Arduino, these are only defined when the appropriate build arg
is set, pointing at the Android NDK path.

Targets are defined for each CPU target at each optimization level,
e.g. including:

  arm_android_debug
  arm64_android_size_optimized
  x64_android_speed_optimized
  x86_android_debug

Top-level targets are defined for each CPU target for the default
optimization level:

  arm_android
  arm64_android
  x64_android
  x86_android

And a top-level "android" target is defined that builds for each
CPU target at the default optimization level.

The default Android backends are set in the Android toolchain, and
default configs were updated.

Tested: Followed instructions in docs with NDK R25C
Change-Id: I4e9636b91a5e4f5c77e2f5a8d389dcd970cf8184
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/55344
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Chinchilla <cachinchilla@google.com>
Commit-Queue: Carlos Chinchilla <cachinchilla@google.com>
Reviewed-by: Keir Mierle <keir@google.com>

BUILD.gn

@@ -15,6 +15,7 @@
 import("//build_overrides/pi_pico.gni")
 import("//build_overrides/pigweed.gni")
 
+import("$dir_pw_android_toolchain/android.gni")
 import("$dir_pw_arduino_build/arduino.gni")
 import("$dir_pw_build/coverage_report.gni")
 import("$dir_pw_build/host_tool.gni")
@@ -261,6 +262,24 @@ group("static_analysis") {
   }
 }
 
+if (pw_android_toolchain_NDK_PATH != "") {
+  group("android") {
+    deps = []
+    foreach(_cpu, pw_android_toolchain_cpu_targets) {
+      _toolchain_prefix = "$dir_pigweed/targets/android:${_cpu}_android_"
+      deps += [
+        ":pigweed_default($_toolchain_prefix$pw_DEFAULT_C_OPTIMIZATION_LEVEL)",
+      ]
+    }
+  }
+
+  foreach(_cpu, pw_android_toolchain_cpu_targets) {
+    _build_pigweed_default_at_all_optimization_levels("${_cpu}_android") {
+      toolchain_prefix = "$dir_pigweed/targets/android:${_cpu}_android_"
+    }
+  }
+}
+
 group("docs") {
   deps = [ "$dir_pigweed/docs($dir_pigweed/targets/docs)" ]
 }

pw_android_toolchain/toolchains.gni

@@ -23,21 +23,18 @@ pw_toolchain_android = {
     name = "android_debug"
     defaults = {
       default_configs = _android + [ "$dir_pw_build:optimize_debugging" ]
-      remove_default_configs = [ "$dir_pw_build:relative_paths" ]
     }
   }
   speed_optimized = {
     name = "android_speed_optimized"
     defaults = {
       default_configs = _android + [ "$dir_pw_build:optimize_speed" ]
-      remove_default_configs = [ "$dir_pw_build:relative_paths" ]
     }
   }
   size_optimized = {
     name = "android_size_optimized"
     defaults = {
       default_configs = _android + [ "$dir_pw_build:optimize_size" ]
-      remove_default_configs = [ "$dir_pw_build:relative_paths" ]
     }
   }
 }

pw_containers/BUILD.gn

@@ -17,6 +17,7 @@ import("//build_overrides/pigweed.gni")
 import("$dir_pw_bloat/bloat.gni")
 import("$dir_pw_build/target_types.gni")
 import("$dir_pw_docgen/docs.gni")
+import("$dir_pw_toolchain/traits.gni")
 import("$dir_pw_unit_test/test.gni")
 
 config("public_include_path") {

targets/android/target_docs.rst

@@ -9,3 +9,58 @@ Native Development Kit (NDK).
 .. warning::
   This target is under construction, not ready for use, and the documentation
   is incomplete.
+
+Setup
+=====
+You must first download and unpack a copy of the `Android NDK`_ and let Pigweed
+know where that is located using the ``pw_android_toolchain_NDK_PATH`` build
+arg.
+
+.. _Android NDK: https://developer.android.com/ndk
+
+You can set Pigweed build options using ``gn args out``.
+
+Building
+========
+To build for this Pigweed target, simply build the top-level "android" Ninja
+target. You can set Pigweed build options using ``gn args out`` or by running:
+
+.. code:: sh
+
+  gn gen out --args='
+    pw_android_toolchain_NDK_PATH="/path/to/android/ndk"'
+
+On a Windows machine it's easier to run:
+
+.. code:: sh
+
+  gn args out
+
+That will open a text file where you can paste the args in:
+
+.. code:: text
+
+  pw_android_toolchain_NDK_PATH = "/path/to/android/ndk"
+
+Save the file and close the text editor.
+
+Then build with:
+
+.. code:: sh
+
+  ninja -C out android
+
+This will build Pigweed for all supported Android CPU targets at the default
+optimization level, currently arm, arm64, x64, and x86.
+
+To build for a specific CPU target only, at the default optimization level:
+
+.. code:: sh
+
+  ninja -C out arm64_android
+
+Or to build for a specific CPU target and optimization level:
+
+.. code:: sh
+
+  ninja -C out arm64_android_size_optimized

targets/android/target_toolchains.gni

@@ -28,8 +28,27 @@ if (pw_android_toolchain_NDK_PATH != "") {
 
     # Facade backends
     pw_assert_BACKEND = dir_pw_assert_basic
+    pw_chrono_SYSTEM_CLOCK_BACKEND = "$dir_pw_chrono_stl:system_clock"
+    pw_chrono_SYSTEM_TIMER_BACKEND = "$dir_pw_chrono_stl:system_timer"
     pw_log_BACKEND = dir_pw_log_basic
+    pw_sync_BINARY_SEMAPHORE_BACKEND =
+        "$dir_pw_sync_stl:binary_semaphore_backend"
+    pw_sync_CONDITION_VARIABLE_BACKEND =
+        "$dir_pw_sync_stl:condition_variable_backend"
+    pw_sync_COUNTING_SEMAPHORE_BACKEND =
+        "$dir_pw_sync_stl:counting_semaphore_backend"
+    pw_sync_MUTEX_BACKEND = "$dir_pw_sync_stl:mutex_backend"
+    pw_sync_TIMED_MUTEX_BACKEND = "$dir_pw_sync_stl:timed_mutex_backend"
+    pw_sync_INTERRUPT_SPIN_LOCK_BACKEND = "$dir_pw_sync_stl:interrupt_spin_lock"
+    pw_sync_THREAD_NOTIFICATION_BACKEND =
+        "$dir_pw_sync:binary_semaphore_thread_notification_backend"
+    pw_sync_TIMED_THREAD_NOTIFICATION_BACKEND =
+        "$dir_pw_sync:binary_semaphore_timed_thread_notification_backend"
     pw_sys_io_BACKEND = dir_pw_sys_io_stdio
+    pw_thread_ID_BACKEND = "$dir_pw_thread_stl:id"
+    pw_thread_SLEEP_BACKEND = "$dir_pw_thread_stl:sleep"
+    pw_thread_THREAD_BACKEND = "$dir_pw_thread_stl:thread"
+    pw_thread_YIELD_BACKEND = "$dir_pw_thread_stl:yield"
 
     pw_build_LINK_DEPS = []
     pw_build_LINK_DEPS += [