Respect the rollback data policy (3/n)

1. Don't snapshot user data when the policy is WIPE.
2. Add tests to verify user data is wiped/restored correctly.

Bug: 144683152
Test: atest RollbackStoreTest RollbackUnitTest AppDataRollbackHelperTest
Test: atest RollbackTest StagedRollbackTest
Change-Id: I470233080efbba724ace6f0dbf2ab5256db49054