TEMPORARY: Reintroduce button-backlight (and respective inactivity timeout)

The power manager rewrite from Change I1d7a52e98f0449f76d70bf421f6a7f245957d1d7
completely removed support for control of the button backlights, which makes
all capacitive buttons out there stay dark. The commit message in that change
mentions it hasn't been implemented _yet_, so this fix should be temporary
until upstream does their own implementation

Change-Id: I6094c446e0b8c23f57d30652a3cbd35dee5e821a
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.