AudioService: device connection intent: never send null action

Change 145c9534 removed a test on the type of device for which
  a ACTION_HEADSET_PLUG intent is broadcast. As a result, for some
  devices (e.g. DEVICE_OUT_USB_DEVICE), there is a code path in
  AudioService.sendDeviceConnectionIntent() where the intent's
  action is not set, causing a WTF exception to be thrown for
  the broadcast.
The fix consists in always checking that the intent will be sent
  with a non-null action.

Test: connect a USB device recognized as DEVICE_OUT_USB_DEVICE
Bug: 64724117
Change-Id: Ib2a486d1fabbde58d6a8f8370d5803c694977ee1