Fix Android release build configuration

[Harzva]

Summary

• build release APKs from the checked-in Android project so MobileCode native Phone Use, Alpine sandbox, and HTML renderer sources are preserved
• build the pure release flavor and stage the matching APK output
• make regenerated Android projects enable core-library desugaring required by flutter_local_notifications

Validation

• python3 -m py_compile mobile_agent/tooling/prepare_android_project.py
• actionlint .github/workflows/android-apk.yml .github/workflows/mobile-app-release.yml
• clean temporary checkout-equivalent build: flutter build apk --release --flavor pure --dart-define=MOBILECODE_BUILD_CHANNEL=pure --target lib/main.dart

Fixes the failed v0.1.69 Android release job (checkReleaseAarMetadata: core library desugaring disabled).