PowerManager: Re-integrate button brightness

This is a squash of the following commits:

Author: Danny Baumann <dannybaumann@web.de>
Date:   Thu Aug 22 08:53:24 2013 +0200

    Add PowerManager integration for button and keyboard backlight.

    Allows setting button and keyboard backlight brightness as well as
    button timeout.

    Change-Id: I550cccafc0a8f90d6347de9261adb26b75955cc4

Author: nadlabak <pavel@doshaska.net>
Date:   Sun Jun 7 02:01:05 2015 +0200

    PowerManagerService: Fix updating of mUserActivitySummary

    I7f1fc35a1573717d1ea101a07c4171d6f66d1553 missed the fact that the primary
    purpose of the affected condition block was to update mUserActivitySummary
    and the button/keyboard light handling was just appended to it later.

    This fixes the waking from dream/screensaver by user activity.

    I30c5c8c9c09e3d57ace18cac72b783510b9b3bf3 is removed here as well as it was
    just a band aid.

    jira: NIGHTLIES-1285

    Change-Id: I6b2f6c58e73110787d62e86d4d2ef538638cf491

Author: Bruno Martins <bgcngm@gmail.com>
Date:   Tue Dec 26 17:15:05 2017 +0000

    Forward-port button brightness implementation to O

     * Reworked for the new handler interface, restoring also removed
       methods (partial revert of commit 86c39f9e).

     * Keyboard backlight brightness support left out for now.

Change-Id: Ib22b727ed79752e3ae4666993e54b82aecd8d341