USB: g_mass_storage: Always allow disabling mass storage by writing to lun file

For android builds we disable the check for curlun->prevent_medium_removal.
Instead we let the framework manage unmounting policy, as we sometimes need
to unmount after the media has been removed.
This also helps support hosts that do not inform the device when the media
has been unmounted.

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