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 option for switching between UMS and MTP/PTP mode. (1/2)

Change-Id: I8e692d468cb36ccc97ca69b8a2eb11afeb711eae

This adds support for switching between UMS and MTP/PTP.
Framework part (this part) will only enable UMS if the config setting is set.
Standard is to enable UMS if the primary volume supports UMS.

It's only porting this http://review.cyanogenmod.org/#/c/21117/
Merge remote branch 'cm/cm-10.1' into cm-10.1