Only show Mount USB storage if UMS possible

In the mounts and storage menu:
* Reduce the complexity of if/elses for /data/media and UMS specific
  menu entries with a simple struct
* Only show UMS mount option if vold managed volumes are available and
  usb-otg is not in use. Vold does not have any mechanism to determine
  if a managed volume is usb-otg, so we have to rely on string
  comparison to the volume label. "usbdisk" is by far the most common
  label, so this should cover most devices just fine.

Change-Id: I8a203a614e1c603c32a06b1afb77834cc4cd51ce