PlaylistControl: Similar to MPDControl, add a class to send playlist commands.

Similar to MPDControl, add a class to send simple threaded playlist commands
to clean up PlaylistFragment.
PlaylistFragment: Avoid return until the end of the method.
PlaylistFragment: Remove unused AsyncTask.

If not doing any work in the background, AsyncTask has no use.
PlaylistFragment: Cleanup the update() method.
PlaylistFragment: Fix warnings.
PlaylistFragment: Small cleanup for onActionItemClicked().
NowPlayingFragment: Cleanup popup menu generation.
NowPlayingFragment: Remove some fields.