Fix Bluetooth control button unresponsiveness.

With Oreo and later, Bluetooth control buttons may kill FFA if it is not the foreground application. Once this happens to resume playback, one needs to restart playback from the phone, rather than the play/pause action of Bluetooth headset.

For example: D MediaSessionService: Sending KeyEvent { action=ACTION_UP, keyCode=KEYCODE_MEDIA_PLAY, scanCode=0, metaState=0, flags=0x0, repeatCount=0, eventTime=0, downTime=0, deviceId=-1, source=0x0 } to audio.funkwhale.ffa.dev/audio.funkwhale.ffa.dev (

W ActivityManager: Background start not allowed: service Intent { act=android.intent.action.MEDIA_BUTTON cmp=audio.funkwhale.ffa.dev/audio.funkwhale.ffa.playback.PlayerService (has extras) } to audio.funkwhale.ffa.dev/audio.funkwhale.ffa.play

549 uid=10149 pkg=audio.funkwhale.ffa.dev startFg?=false D AndroidRuntime: Shutting down VM --------- beginning of crash E AndroidRuntime: FATAL EXCEPTION: main E AndroidRuntime: Process: audio.funkwhale.ffa.dev, PID: 14549 E AndroidRuntime: java.lang.IllegalStateException: Not allowed to start service Intent { act=android.intent.action.MEDIA_BUTTON cmp=audio.funkwhale.ffa.dev/audio.funkwhale.ffa.playback.PlayerService (has extras) }: app is in background uid UidRecord{72fa8f8 u0a149 CAC bg:+11m56s597ms idle change:cached procs:1 seq(0,0,0)} E AndroidRuntime: at android.app.ContextImpl.startServiceCommon(ContextImpl.java:1577) E AndroidRuntime: at android.app.ContextImpl.startService(ContextImpl.java:1532) E AndroidRuntime: at android.content.ContextWrapper.startService(ContextWrapper.java:664) E AndroidRuntime: at audio.funkwhale.ffa.playback.MediaSession$connector2.invokelambda-3lambda-2(MediaSession.kt:47) E AndroidRuntime: at audio.funkwhale.ffa.playback.MediaSessionconnector2.r8lambdajU84j_zRyeYuvwLrRY0b6XyQBMs(Unknown Source:0) E AndroidRuntime: at audio.funkwhale.ffa.playback.MediaSession$connector2$$ExternalSyntheticLambda0.onMediaButtonEvent(Unknown Source:2) E AndroidRuntime: at com.google.android.exoplayer2.ext.mediasession.MediaSessionConnectorComponentListener.onMediaButtonEvent(MediaSessionConnector.java:1396) E AndroidRuntime: at android.support.v4.media.session.MediaSessionCompatCallbackMediaSessionCallbackApi21.onMediaButtonEvent(MediaSessionCompat.java:1602) E AndroidRuntime: at android.media.session.MediaSessionCallbackMessageHandler.handleMessage(MediaSession.java:1471) E AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:106) E AndroidRuntime: at android.os.Looper.loop(Looper.java:193) E AndroidRuntime: at android.app.ActivityThread.main(ActivityThread.java:6718) E AndroidRuntime: at java.lang.reflect.Method.invoke(Native Method) E AndroidRuntime: at com.android.internal.os.RuntimeInitMethodAndArgsCaller.run(RuntimeInit.java:491) E AndroidRuntime: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) W ActivityManager: Force finishing activity audio.funkwhale.ffa.dev/audio.funkwhale.ffa.activities.MainActivity

xref: https://stackoverflow.com/questions/46445265/android-8-0-java-lang-illegalstateexception-not-allowed-to-start-service-inten