BluetoothDeviceManager.java: add null check before call mBluetoothAdapter.getActiveDevices

As the bluetoothAdapter might be null, we need to check first before
we call mBluetoothAdapter.getActiveDevices like before.
Otherwise we would get following crash errors when there is no bluetooth feature
with the platform.

    10-09 16:12:33.654   953   953 E AndroidRuntime: *** FATAL EXCEPTION IN SYSTEM PROCESS: main
    10-09 16:12:33.654   953   953 E AndroidRuntime: java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.List android.bluetooth.BluetoothAdapter.getActiveDevices(int)' on a null object reference
    10-09 16:12:33.654   953   953 E AndroidRuntime:    at com.android.server.telecom.bluetooth.BluetoothDeviceManager.getUniqueConnectedDevices(BluetoothDeviceManager.java:160)
    10-09 16:12:33.654   953   953 E AndroidRuntime:    at com.android.server.telecom.bluetooth.BluetoothRouteManager.getConnectedDevices(BluetoothRouteManager.java:611)
    10-09 16:12:33.654   953   953 E AndroidRuntime:    at com.android.server.telecom.CallAudioRouteStateMachine.getInitialAudioState(CallAudioRouteStateMachine.java:1810)
    10-09 16:12:33.654   953   953 E AndroidRuntime:    at com.android.server.telecom.CallAudioRouteStateMachine.initialize(CallAudioRouteStateMachine.java:1524)
    10-09 16:12:33.654   953   953 E AndroidRuntime:    at com.android.server.telecom.CallsManager.<init>(CallsManager.java:524)
    10-09 16:12:33.654   953   953 E AndroidRuntime:    at com.android.server.telecom.TelecomSystem.<init>(TelecomSystem.java:322)

Test: boot tested with x15 build

Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
Change-Id: Ib42ba98211dc74dcc59fab8171e6a41795161334