MediaRouter bluetooth a2dp selection

Remove bluetooth permission check from internal AudioService method
setBluetoothA2dpOn

Manage BT A2DP state in MediaRouter. A2DP is only enabled or disabled
when either the system built-in audio route or the A2DP audio route is
selected; when selecting any other route the current state is left
alone.

Change-Id: Ib14274e206e79bd8762edca1205ecfa87b7a94cf