Handle failures after partial mounts

When we try mounting an EmulatedVolume, we may mount sdcardfs but fail
in any of the FUSE mounts, in this case we should unmount whatever
mounts we made during the mount.

Test: Intentionally causing a partial failure, verified that sdcardfs
gets unmounted
Bug: 147610762

Change-Id: I29ed044ed8ab8aa3dd83bc97a49eb3140ce4fe27