[codex] fix Android native alarm flow

[jjoonleo]

Summary

• Add Android full-screen native alarm activity and AlarmManager launch plumbing.
• Improve alarm reconciliation, fallback handling, missed-alarm catch-up, and status reporting.
• Route native alarm launches through /scheduleStart before the active preparation flow.
• Fix Google Sign-In ID token retrieval by configuring the server client ID and removing a duplicate listener.

Why

Android alarms were firing through notification-style behavior and the start action could land on home or skip the schedule-start prompt. This makes the Android alarm flow closer to the intended native alarm experience while preserving the existing Flutter preparation flow.

Validation

• ./gradlew :app:compileDebugKotlin
• flutter test test/presentation/app/bloc/schedule/schedule_bloc_test.dart
• flutter test test/presentation/alarm/screens/preparation_flow_widget_test.dart
• flutter analyze

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