UMS [2/6]: Add option for switching between UMS and MTP/PTP mode.

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.

1: https://gerrit.omnirom.org/#/c/5041/
3: https://gerrit.omnirom.org/#/c/5043/
4: https://gerrit.omnirom.org/#/c/5044/
5: https://gerrit.omnirom.org/#/c/5045/
6: https://gerrit.omnirom.org/#/c/5046/

Signed-off-by: Humberto Borba <humberos@gmail.com>
Change-Id: I8e692d468cb36ccc97ca69b8a2eb11afeb711eae
Merge "UMS [2/6]: Add option for switching between UMS and MTP/PTP mode." into kitkat