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

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.

p2: Fix compile error
p3: Some code styling
p4: Fix mysterious bug where USB notification is not fired when switching from UMS.
p5: Revert last commit.

Change-Id: I753b7dd3823958da1b043ca44b50118d67637cc1