Revise BT active device detection

When figuring out which BT device is currently routing audio, use the
getActiveDevice(s) APIs provided by BT and use the last-received
ACTIVE_DEVICE_CHANGED broadcast as a fallback in case multiple active
devices are detected across profiles

Bug: 122359801
Test: unit, manual
Change-Id: Id5ae17abcee023ddb9172968b9def7de38d23c63