Added support for USB audio devices

Two types of USB audio devices are defined:
- USB audio device: the audio device in USB device mode while
the Android device is in USB host mode.
- USB audio accessory: the audio device in USB host mode while
the Android device is in USB device mode.

Renamed intents for analog and digital docks to avoid confusion:
 - ACTION_USB_ANLG_HEADSET_PLUG to ACTION_ANALOG_AUDIO_DOCK_PLUG
 - ACTION_USB_DGTL_HEADSET_PLUG to ACTION_DIGITAL_AUDIO_DOCK_PLUG

Factorized code in AudioService broadcast receiver.

Change-Id: I1b6d0257a9d68ecb9495c78c98bac8c67fec7891