SystemUI: Port statusbar brightness control

This is a squash of the following changes:

Author: Danesh Mondegarian <daneshm90@gmail.com>
Date:   Sun Nov 10 23:30:56 2013 -0800

    SystemUI : Port forward statusbar brightness control

    Brings back the status bar brightness control from cm-10.2/cm-11

    Change-Id: Icc04e7da3286e9925f8b90528c1e022b33d33e9d

Author: Roman Birg <roman@cyngn.com>
Date:   Mon Dec 1 16:08:21 2014 -0800

    SystemUI: improve brightness slider behavior

    Mimic the brightness slider behavior in the statusbar.
    This adds logic to make the statusbar slider also work with automatic
    brightness mode enabled and it will instead adjust the temporary automatic
    brightness overrride.

    Change-Id: I78814a785decc83df94844fcc2f489c8cbec2b65
Signed-off-by: Roman Birg <roman@cyngn.com>

Author: Pawit Pornkitprasan <p.pawit@gmail.com>
Date:   Sun Dec 28 20:42:41 2014 +0700

    SystemUI: status bar brightness: store value as int

    Non-automatic brightness value is stored as int, not float.

    Symptom: adjust the brightness in the status bar, the brightness
             slider in the notification bar will always be set to full

    Change-Id: I5f9a6e97bae2c44c3aff7c9222186a6dbda5d902

Author: Zhao Wei Liew <zhaoweiliew@gmail.com>
Date:   Fri Oct 7 10:09:04 2016 +0800

    SystemUI: statusbar: Fix status bar brightness control feature

    Bring in the missing fixes from CM 13.

    This commit squashes the following commits from CM 13.0:
    6397527c SystemUI: fix status bar brightness slider eating events
    bbef4227 SystemUI : fix brightness slider for automatic brightness

    Change-Id: I85e0b99dc33468b36222128660707fe481cf8b33

Author: Zhao Wei Liew <zhaoweiliew@gmail.com>
Date:   Fri Oct 7 08:56:25 2016 +0800

    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

Change-Id: Ic8ded146dc8605e6140f4352cafa6f961ad3decc