Update ringer to take BT device into account

When there is an HFP device connected but the phone would otherwise not
play an audible ringtone, acquire audio focus anyway so that music won't
play over top of the BT device-generated sound.

Change-Id: I898dddfb29147debf5d520583bed02d2012b6cb7
Fixes: 34084026
Test: unit tests and manual