Fix android:exported for gradle/RN0.68

Execution failed for task ':react-native-track-player:processDebugManifest'.
> A failure occurred while executing com.android.build.gradle.tasks.ProcessLibraryManifest$ProcessLibWorkAction
   > Manifest merger failed : android:exported needs to be explicitly specified for <receiver>. Apps targeting Android 12 and higher are required to specify an explicit value for `android:exported` when the corresponding component has an intent filter defined. See https://developer.android.com/guide/topics/manifest/activity-element#exported for details.