Methods to translate sandbox paths.

System components (like MediaProvider) will live in a mount namespace
that has a view of the "real" shared storage device, and normal apps
will have "sandboxed" views of the shared storage device.  (Parallel
changes are implementing these namespaces in vold and installd.)

The system components mentioned above will need to translate between
the two namespaces, so this change introduces methods that perform
that translation, along with a nice batch of tests to verify.

Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/StorageManagerServiceTest.java
Bug: 111893193, 111892833, 111268862
Change-Id: Iae91a44ce09eb33d6cd9b90f6c7b4f88c8cd12f0