Prevent unnecessary call of setBluetoothA2dpOn(false)

When an active BT device is changed, instead of just notifying that
the changed BT name, AudioService sometimes notify it with two steps.

1) Notify prev BT disconnected
2) Notify new BT connected

By not calling setBluetoothA2dpOn(false) when BT is disconnected,
we can prvent the unnecessary call of it.

Bug: 144784716
Test: manual
Change-Id: I31442204cd4dd4a78552e0f993e9dfa37d6fc03f