Older versions of Android use toolchains that only accept c++2a.
Replace 'c++20' with 'c++2a' in Android.bp to be compatible with more
Android versions.
Change-Id: Icacf96d91ea79060a4e2df53ccb53bf6500e8223
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/100180
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Vic Yang <victoryang@google.com>