Fix ringing audio focus handling.

Test case:
- enable increasing ringtone starting at volume 0
- start any music player
- get an incoming call

Previous behaviour:
When ringtone volume is increased from 0 to 1, music continues playing
even
though ring tone is playing as well.

New behaviour:
When ringtone starts, music is attenuated to make user aware of incoming
call. When ringtone volume is increased to 1, music stops playing.

Change-Id: Ifc32ea162d4527572be0574024116d10ee9bca66