Add support for local audio tracks

[anilbeesetti]

Summary

• add a local audio picker alongside subtitle selection while keeping every selected audio file available as a track
• persist external audio URIs and restore them across playback sessions
• merge external audio media sources securely and refresh the current source without losing playlist state
• constrain merged playback to the primary video duration so longer audio cannot delay exit or playlist advancement
• add database migration, persistence, command, attachment, picker, and media-source regression coverage

Testing

• ANDROID_SERIAL=emulator-5554 ./gradlew ktlintCheck :feature:player:connectedDebugAndroidTest :app:assembleDebug
• 26 player instrumentation tests passed on an Android 16 Pixel 6a read-only emulator
• verified generated 4-second video playback ends at the video boundary with a generated 9-second external audio track
• installed the verified arm64 debug APK on the requested wireless adb device