app/jni: Clean up unused JNI global refs
app/jni: Clean up unused JNI global refs

mpv-android/mpv-android,jeeb/mpv-android,jeeb/mpv-android,jeeb/mpv-android,mpv-android/mpv-android,jeeb/mpv-android,jeeb/mpv-android,mpv-android/mpv-android,mpv-android/mpv-android,mpv-android/mpv-android