Fix errors that could lead to brightness flickering

Ensures that changing the brightness doesn't lead to
another write to settings from SystemUI. Also makes
sure that a BrightnessDialog from a background user
is never showing. While we're in there, fix the obsolete
window flags as well and ensure BrightnessDialog only ever
allocates one BrightnessController.

Bug: 16297345
Change-Id: Id95b598b0fa99680072bc160afb00d2cd49f05ca