Protect runtime storage mount points.

We have a bunch of magic that mounts the correct view of storage
access based on the runtime permissions of an app, but we forgot to
protect the real underlying data sources; oops.

This series of changes just bumps the directory heirarchy one level
to give us /mnt/runtime which we can mask off as 0700 to prevent
people from jumping to the exposed internals.

Also add CTS tests to verify that we're protecting access to
internal mount points like this.

Bug: 22964288
Change-Id: I32068e63a3362b37e8ebca1418f900bb8537b498