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
Fixed FC on calling unregisterReceiver with non registered receiver arg.

Crash Log: http://pastebin.com/r0qSGvbb

Patchset 1: Fixed a tab.

Patchset 2: Fixed spacing.

Change-Id: Idd3a7d28d62a980e5d8841535b0c9b57ea6e6a01