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

Change-Id: I6cf50a41a9a82e40e702f5231ce663636c149613

This adds support for switching between UMS and MTP/PTP.
Settings part (this part) adds option in USB settings (statusbar/notification drawer) for enabling UMS,
and adds setting in Storage menu to enable / disable UMS, this since the USB settings page disappears when enabling UMS.

It's only porting this http://review.cyanogenmod.org/#/c/21115/

Conflicts:
	res/values/strings.xml