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
1 file changed