Move all the permissions check up for unmount

We have already scanned the state of the OBB in mountObb, so check the
caller against the stored state in the unmountObb call. This allows us
to ensure the calling binder is the same one that mounted it since we
tie the lifecycle of the OBB to the lifecycle of the binder.

Change-Id: I45d9cfbab5d3f5b37a6a9b594b10bd8b91cccc45