AudioManager: restore setBluetoothA2dpOn() method

Remove deprecation on setBluetoothA2dpOn() method so that applications
can override the default audio policy which is to use A2DP
for media whenever connected.

The request is not persistent and the default policy is restored when a
new A2DP i ro wired headset connection occurs.

Bug 6485897.

Change-Id: I2a4b6b6bdba55f7b133e64f86d27c03eb86acfa4