MPD.java: Fix Add & Play.

This fixes a small bug added (abd63e8) where Add & Play broke.
widgets: Use the broadcast receiver for receiving MPD commands.

This further cleans up the WidgetHelperService until someone gets around to
giving it some much needed love.
MPDControl: Use a connection lock.

Didn't understand connection locks before, and I did things the hard way.
This also blocks the thread up to 5 seconds waiting for a valid connection.