Add separate permission and group for access to MTP USB driver

Since the "usb" group and ACCESS_USB permission will be used by regular apps
to access USB devices in host mode, we need a separate permission for the
device side MTP driver.

Change-Id: Ibb546b8726d61e041fc7c273f71929624ad1575b
Signed-off-by: Mike Lockwood <lockwood@android.com>