PowerManagerService: Implement setButtonBrightnessOverrideFromWindowManager method

Differs from Android 4.1 (and earlier) implementation in that the
overridden button light still turns off after BUTTON_ON_DURATION instead of
staying on until screen off.  However, button lights are annoying, and
really the purpose of this is to allow apps (e.g., Screen Filter) to
_disable_ the button lights altogether.
Add a long-click target (display settings) to the brightness tile

Change-Id: I538cda0ecf999467d7b6a30501807c0c52fb59b6