SystemUI: Port brightness slider changes

 * Expose AutoBrightness button
 * Add content observers
 * Use vector drawables for brightness icons

[BadDaemon]
 * Adapt to SystemUI changes
 * Change code style to use lambdas and locals where possible

Author: Luca Stefani <luca020400@lineageos.org>
Date:   Thu Apr 20 18:00:52 2017 +0200

    SystemUI: Use AOSP AutoBrightness implementation

    Change-Id: I3e3660065407d460f30c6e936d951d4d5eae30c6

Author: Michael Bestas <mikeioannina@gmail.com>
Date:   Thu Apr 27 14:57:49 2017 +0300

    SystemUI: Use already existing overlay to show QS auto brightness

    * No need for a new overlay when one exists for years

    Change-Id: I9e09a1770f8e645093e880cf1a48597405e0bcf2

Author: Alberto97 <albertop2197@gmail.com>
Date:   Mon May 29 21:07:05 2017 +0200

    SystemUI: Specify user while changing brightness mode

    Allow any user changing brightness mode,
    without this only the main one is able to do that.

    BUGBASH-598

    Change-Id: Ib51e9a45c42837cabb1cfb0d48334165b023a827

Author: Michael Bestas <mkbestas@lineageos.org>
Date:   Sun Apr 30 13:45:39 2017 +0300

    SystemUI: Add tunable for auto brightness icon

    Change-Id: I02fe3f5500f95ba9369b952bbec2a073b7ae8316

Author: Alexander Westphal <westphal.alex94@gmail.com>
Date:   Wed Apr 26 09:20:53 2017 +0100

    SystemUI: use vector drawables for brightness icons

    Change-Id: I8d22f8de22040d68ed1bfab0da3a78aec717fa62

Author: qjohn <townex22@gmail.com>
Date:   Sun Jun 18 16:28:10 2017 +0700

    SystemUI: Remove unnecessary brightness icons

    Change-Id: I7e60ff73125bd0304f9ca14ad5c5a9ce11ffceda

Author: Michael W <baddaemon87@gmail.com>
Date:   Mon May 25 15:38:33 2020 +0200

    SystemUI: Brightness slider: Always show the brightness icon

    * Right now, we reserve the space for the icon but it's not displayed
      (reserved space is there so the slider doesn't jump)
    * Always display the icon (if automatic brightness is available)
      so it's more consistent

    Change-Id: I5238dc6aad51407349a8d42fae840ea76841dc3d

Author: Michael W <baddaemon87@gmail.com>
Date:   Fri May 29 20:09:18 2020 +0200

    SystemUI: Fix for auto brightness icon being shown when disabled

    * With commit 9d42c48c the case of a
      manually disabled auto brightness icon got lost
    * Re-implement it by also respecting the Settings value instead of just
      the build time flag

    Change-Id: I991977be7e1f7c33947073dbd3ce7a800fb158bf

Change-Id: Ic72b0084a4108fb4fa26e893bd6d1eaca96cbb6c