Set service priority of A2DP and Headset from BluetoothService via IPC call

We used to do the priority change in the 2 services after the broadcasted
intent reach them. But that left a small time gap that could reject
incoming connection due to undefined priorities.

Bug 4096186

Change-Id: I9bb6aedc7ed98c53a9b00c48eedd20b0cf5f1660