Add metadata and player status information to AVRCP plugin

by João Paulo Rechi Vita for BlueZ

Today the AVRCP plugin in bluetoothd supports only remote control commands. Support for metadata and player status information is missing to fully comply with AVRCP 1.3. This is specially useful for devices with displays, like fancy headsets, cellphones, and car-kits. One of the most illustrative use-case of this feature is a user controlling a media player on his phone via a bluetooth car-kit that shows meta-information about the media beeing played.