Measure and clean up new "sandbox" directories.

The new storage model in Q creates "sandbox" directories that belong
to specific apps.  To keep existing CTS working, we need to measure
and clean up these sandboxes.

Future changes will add support for shared UIDs.

Bug: 112545973
Test: atest android.appsecurity.cts.StorageHostTest
Change-Id: I79fb44982ed97a33f1c6e61d74837bfe54a9a651