[Android] Honor standard SDK environment variables in build_android_library.sh

[mergennachin]

Problem

scripts/build_android_library.sh currently derives Gradle ANDROID_HOME only from ANDROID_SDK, with /opt/android/sdk as the fallback. GitHub Android runners and many developer environments set ANDROID_HOME or ANDROID_SDK_ROOT instead, so the script can ignore the configured SDK and point Gradle at the wrong path.

This was found while validating the Android documentation in #22620. The implementation change is intentionally excluded from that documentation-only PR.

Proposed change

Resolve the SDK path from ANDROID_SDK, then ANDROID_HOME, then ANDROID_SDK_ROOT, with the existing fallback. Pass the resolved path to Gradle through both standard variables.

Test plan

• Exercise the script with each supported environment variable independently.
• Run the Android AAR build job.
• Run the Android emulator integration job.

cc @larryliu0820 @GregoryComer @kirklandsign @cbilgin