MPDroid: Cleanup volume key strokes, use isLocalAudible.

When tracking volume key strokes, don't track if it is probable that the
server controlled by this app will be playing audio on the same device.
PlaylistFragment: Reformat.
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.
NowPlayingFragment: Use MPDCommand.[MIN|MAX]_VOLUME for volume validation.
NowPlayingFragment: Convert onSharedPreferenceChanged() to a switch.
SimpleLibraryActivity: Handle volume buttons the same as MainMenuActivity.