Mount package sandboxes during process initialization.

vold creates external storage sandboxes for packages at
/mnt/user/<user-id>/package/<package-name> when the user starts.
These package specific sandboxes will be mounted at /storage
whenever a new process starts.

Bug: 111890351
Test: n/a (changes are protected by a sys property, there's no change in
      behavior without turning on that property)

Change-Id: I8db4ab823c8dccc454e8004b119ba394a08d6475