Switch sdcardfs over to bind mounts.

Sdcardfs has been modified to use bind mounts
in place of three separate mounts. This lowers
resource usage, and removes a number of race
conditions around updates via differnt views.

Test: Run storage cts tests with sdcardfs enabled
Change-Id: Icdc196ba3d6f46d54d27ef91c01ffebcf81dd47e