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/