Settings: fix bad MTP uncheck behavior

Unchecking MTP will cause it to attempt to
set MTP again which never fires the callback
to re-enable the UI state.  So if the user unchecks it
don't disable UI.

Change-Id: I6d6f3f13cc91fe4df0126b1cf43e571ccb42d62c
(cherry picked from commit 74b88fb3)