StorageManagerTest: assert that the contents of mounted OBBs are readable

When an application mounts an OBB file, the app should be able to
read the contents of the OBB. Ensure that's the case.

(cherrypicked from commit 4fd39adbc330922e15fd94ed94ba4f3ab2ff3af9)

Bug: 17633509
Change-Id: I2018b63b0236ce6b5aee4094e40473315b1948c3
Merge "StorageManagerTest: assert that the contents of mounted OBBs are readable"