SystemUI: Port brightness slider changes

 * Allow hiding brightness slider
 * Expose AutoBrightness button
 * Replace TunerService with content observers

Co-authored-by: Alexander Westphal <westphal.alex94@gmail.com>
Co-authored-by: Luca Stefani <luca020400@lineageos.org>
Co-authored-by: Michael Bestas <mkbestas@lineageos.org>
Co-authored-by: Michael W <baddaemon87@gmail.com>
Co-authored-by: Oliver Scott <olivercscott@gmail.com>
Co-authored-by: qjohn <townex22@gmail.com>
Change-Id: Ic72b0084a4108fb4fa26e893bd6d1eaca96cbb6c