Add (un)registerDataRequestObserver API.

This API allows a privileged app to listen to data requests from the WearableSensingService. The app can then provide the requested data using the existing provideData API.

Rate limiting will be added in a follow-up to limit the scope of this
change.

Test: atest CtsWearableSensingServiceTestCases (see the CTS changes in
the same topic)
Bug: 301427767

Change-Id: I39fc7b094e7780a121e08428877baa671c634275