package-manager-native: Add aidl for package change event and observer.

Iorapd, a native service to prefetch files during app startup,
will register it as an observer via the package manager native service.

Bug: 150742983
Test: Make and run on a blueline device and check the observer works.
Change-Id: I98624fe563b83f5dd801f607fa2b31ab6f6998e7