Add unit test for PersistentDataBlockService

The test verifies the existing implementation in order to provide better
confidence for later refactoring. Once the refactoring is done/in
progress, we will be able to clean up the interface so that all the
blocks have consistent API.

There should be no change to the actual service behavior, except that
wipe is now supported in the DSU case.

Test: atest PersistentDataBlockServiceTest
Bug: 303523453
Bug: 303188712
Change-Id: Ida258ed2511330c0bdf2638256c069cd988985a5