Don't start playing if we couldn't acquire audio focus.

This makes us not start playback when we get a 'play' media button event
in the middle of a call for whatever reason.

JIRA:CYAN-1067
Change-Id: If648da95c10716400eeba0b39a24213764370f7a
Merge "Don't start playing if we couldn't acquire audio focus." into cm-10.1