Check for audio focus before enabling audio path

In scenarios where FM has lost audio focus and FM service is running, we
end up enabling FM audio path when A2DP is disconnected. This results in
more than one audio source being played simultaneously. Enable FM audio
path only when FM has audio focus.

CRs-Fixed: 967753
Change-Id: I8d5f28cb9ce3545908362e83e787fcc43394a0ac