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

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.

Forward port from cm-10.2, adjusted for 4.4

Change-Id: I6cf50a41a9a82e40e702f5231ce663636c149613