SystemUI: Use Tuner API for CM settings

Get rid of all the excess code by implementing TunerService.Tunable
and observing any changes made to the settings through it.

Remove UserContentObserver as the Tuner API handles user switches.

Also remove some unused imports that were introduced in earlier
CM commits, but were never removed since.

Change-Id: Iecafafabdaec82b3b3c72293bea865de48f0e90a