Always send KEYCODE_MEDIA_PLAY_PAUSE

Instead of sending _PLAY or _PAUSE as requested by an AVRCP controller,
forward only KEYCODE_MEDIA_PLAY_PAUSE. That way, if the remote device is
not correctly tracking the playback state, the media player will pause
or resume playback in a manner consistent with a users expectations.

Change-Id: Ibc2e542c799fa500f07dbc62408015e74cd6799e
Fixes: 64006398
Test: Manual; play audio; issue PLAY/PAUSE commands from headphones