Use test stubs for RollbackTests, not system stubs.

Adds @TestApi for all system apis needed for RollbackTests.

Removes @SystemApi for RollbackManager.expireRollbackForPackage and
RollbackManager.reloadPersistedData, which were always intended to be
used solely for testing.

Bug: 127282563
Test: atest RollbackTest StagedRollbackTest
Change-Id: I183b8a484b4a08f36fc19d64241ab408d28c64ca