Fix regression.

When turning off BT while connected to an A2DP/HFP headset we can hit the path

BluetoothHeadsetService.onReceive(BluetoothHeadset.STATE_CHANGED) ->
BluetoothHandsfree.audioOff() ->
BluetoothA2dp.resumeSink() ->
which causes resumeSink() to NPE because mAudioDevices.get() returns null.

It's a race between A2DP marking the device as disconnected, and HFP marking
the device as disconnected.

Fix is to NPE check in resumeSink().

Change-Id: I2782ac8c70ea1678d7de5fcd49bff8e03df36f4e