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.

p2: Remove UMS switch when UMS is not enabled in storage_list overlay
p3: Fix some strings being uppercase

Change-Id: I5aa692488e59518b09a31a6eaf7c04e6108ddc1f

Conflicts:

	res/values/strings.xml
	src/com/android/settings/deviceinfo/UsbSettings.java