Update doc for Platform_apis field in DeviceProperties

The doc for Platform_apis is outdated. The property is used by not only android_app, but also android_test and android_test_helper_app

https://cs.android.com/android/platform/superproject/+/master:build/soong/java/app.go;l=382;bpv=1

Next steps:
1. Clean up existing misuse of Platform_apis and Sdk_version in android_test and android_test_helper_app.

2. Add validation check to ensure proper use of Platform_apis and Sdk_version moving forward android_test and android_test_helper_app.
Currently, android_app already has validation in place. https://cs.android.com/android/platform/superproject/+/master:build/soong/java/app.go;l=275;bpv=1;bpt=1
WIP: https://android-review.googlesource.com/c/platform/build/soong/+/2065270

Test: N/A
Change-Id: I934860cdc8b23268d70e205089118c6451d4cf8a