Add completion callbacks to registerContentObserverAsync

Due to possible race conditions, it might be necessary for the caller to
refresh any possible cached values after the content observer registration
completes. This adds that callback to async API.

Bug: 327558308
Test: newly written unit test
Flag: NONE newly added API is not used anywhere
Change-Id: Ic40bbde50b5a28d6868b50b8834e1814da32539c