Revert "Revert "package-manager: Add an observer to package manager on package change.""

This reverts commit cd9997f3.

Reason for revert: This first reverting is because the CL has to be
submitted with ag/10523008, but they didn't even they had the same topic.
Revert the first reverting together with submitting ag/10523008.

Bug: 150742983

Change-Id: I6c8fb640d1334f2114d6d7b01a44be1a202f0bda