feat(exoplayer): retain playback back buffer

[openanibot]

Closes open-ani/animeko#2323

Summary

• configure ExoPlayer with a 30-second back buffer
• retain buffered samples from a keyframe so short backward seeks can reuse already loaded media

Verification

• ./gradlew :mediamp-exoplayer:compileAndroidMain :mediamp-exoplayer:testAndroidHostTest
• Animeko composite build: ./gradlew :app:shared:video-player:compileAndroidMain :app:shared:video-player:testAndroidHostTest
• Animeko composite APK: ./gradlew --no-configuration-cache :app:android:assembleDefaultDebug -Pani.android.abis=arm64-v8a
• Android 15 / API 35 emulator (1344x2992, 480 dpi): installed the composite APK, played a real stream, performed a 5-second backward swipe, and confirmed playback resumed in under one second without an app crash
• Desktop was not run because mediamp-exoplayer is Android-only

Android runtime

The middle frame captures the 5-second backward gesture; the final frame is from the same 6.1-second recording after playback resumed.

Before seek (01:55)	Backward 5 seconds	Playback resumed