Fix issue that music popped up from speaker during ringing call

Pre-condition: Music is played through speaker.
When there is a ringing call, if user presses volume key, music gets
played through speaker. This is caused by NO_MORE_RINGING_CALLS message
is sent to CallAudioModeStateMachine resulting in loss of audio focus.
Fix issue by not sending the message if there is an existing call.

Bug: 31231586
Change-Id: I8317781c8db02baafbd10321b19783cff646ec51