fix: Add ANDROID_HOME auto-detection for self-hosted runner

- Auto-detect Android SDK at common locations (~\nAndroid/Sdk, ~/Library/Android/sdk)
- Export ANDROID_HOME from detection step for use in subsequent steps
- Add fallback ANDROID_HOME in build step
- Better error messages for SDK detection failures
- Ensures gradle can locate Android SDK on self-hosted runners

This fixes the 'SDK location not found' error when running on
self-hosted GitHub Actions runners that don't have ANDROID_HOME
explicitly set in the shell environment.

Author: digitalnomad91