Do not access MTP devices when disabled.

The USB data transfer is disabled we should not allow access MTP devices (e.g.
usb sticks). We have two ways of accessing them: Either by mounting them
or by creating a MTPDevice in an app.

Of course an app could implement implement their own MTPDevice
implementation. In this case we cannot enforce the policy without
completely suppressing all MTP USB devices which would be too
restrictive.

Note: When the policy is set we do _not_ disconnect already connected
MTP devices

Fixes: 31472955
Change-Id: I6080c48c49657102774b2b3b4d89ff030245a266