UsbDeviceManager: fix b/21429947 regression

As discussed in b/21429947 (commit
67401906), MTP must always be
enabled, even if access to the underlying MTP data is disabled.
Otherwise, Android will not enumerate on the USB bus, and won't
receive notifications from the kernel about USB state changes. This
effectively prevents using MTP functionality on user builds, or
on userdebug/eng builds with adb turned off.

Always ensure that MTP is the default driver mode.

Get rid of one use of the persistent property. The persistent property
was already pulled from a number of devices, and as explained in
commit fcf10f7c, the intent was that
the persistent property would only hold the persistent adb state.

Bug: 22447614
Bug: 21429947
Change-Id: I8b3690a1bafb7cea0d5a69d73c1065c7fc64c653