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.

CRs-Fixed: 407992
Change-Id: I914d7961fe06074c09b76d5bbc336d1454598488
Signed-off-by: Chiranjeevi Velempati <cvelempa@codeaurora.org>