Feature: Stop music on call and duck on ringing

[459below]

At the moment jitsi-android doesn't care for music. I would like to implement a behavior as such:

-Duck music while ringing on incoming or outgoing call
-Stop music if the actual call takes place

[459below]

I did a first implementation here which I'm going to start testing/using. I'll do a PR as soon as it can be deemed stable.

[459below]

CyanogenMod's Apollo Music Player doesn't respond to the intent. I grepped through Apollo's code and it doesn't seem to be handling the "com.android.music.musicservicecommand" intent at all. I might have to fake key inputs a la "KeyEvent.KEYCODE_MEDIA_PAUSE" to get it to pause its playback.

Players like AOSP's Music, Google's Play Music and Samsung's Music comply, though.