Add getters/setters for ObserverInternal#packages

Add proper getters and setters for managing the map of
MonitoredPackage objects associated with an observer.

Test: atest StagedRollbackTest
Test: atest NetworkStagedRollbackTest
Test: atest PackageWatchdogTest
Bug: 172238875
Change-Id: I66c9f4b372c78e29591f0263e676be3f99c81439