Add separate permission and group for access to the 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: Id03f7c4f3d2712c489b4b74cf17a72a182084501
Signed-off-by: Mike Lockwood <lockwood@android.com>