Fix null RollbackManager in RollbackHealthObserver and minor todos

1. RollbackManager was instantiated by Context#getSystemService in
   RollbackHealthObserver. At this time, RollbackManager has not yet
   been published as a service to the system, so RollbackManager was null.
   We now #getSystemService lazily so RollbackManager is never null.
2. Pass causePackages from RollbackHealthObserver to RollbackManager
3. Also fixed flaky ignored test exercising PackageWatchdog auto rollback

Test: atest RollbackTest
Bug: 123615508 112431924

Change-Id: I8f15c257b1efd96a96656405d3d7f74576a32c8e