base

Fix exception in AudioService when no BT Headset is connected.

If AudioManager.startBluetoothSco() is invoked with no Bluetooth Headset
connected, a NullPointerException will ensue.

Added check to make sure mBluetoothHeadsetDevice is set before accessing
it.

Bug: 17601845
Change-Id: Ia00bfffbea5484230b11a74787fe3309c18f824d