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>