Fix overactive media routing

For MediaRouterService,
- Delay route restoration to BT for 0.5s in case the app plays
  more audio

For AudioPlaybackMonitor,
- Rename to AudioPlayerStateMonitor.
- Combine two listeners into one.
- Take a handler for the looper of the callback methods.
- Move the registration of playback callback into audio service
  from the constructor to the caller.

Bug: 65376604
Test: manual tests as follows
1. Checked that the lastly played app receives the media key events
although the app's media session doesn't report its playback state.
2. Checked that the lastly played app receives the media key events
although the app's media session is released.
3. Confirmed that this mitigates the issue.
Change-Id: I52e3719cd962f5d92c2f0e547309ce1314cc0926