Another round of changes on Content Capture.

- Get rid of activity-level events.
- Renamed InteractionSessionId and InteractionContext to
  ContentCaptureSessionId and ContentCaptureContext (and made them public)
- Create the explicit concept of ContentCaptureSesssion (and moved notification
  APIs to it).
- Added APIs to let apps create new sessions (not implemented yet).
- Added APIs to remove user data based on some context properties (like URI).

The reasoning behind this change is to let app developers explicitly associate
the captured content with some app-level domain (and also let the app ask the
service to clear such data at user's request). For example, a browser app
(and WebView) can use these APIs to associate the content capture events with
the URL being rendered.

Bug: 117944706
Fixes: 121034139

Test: atest CtsContentCaptureServiceTestCases
Test: m update-api && m

Change-Id: I7841da303b6a39c825651b03a07e3081fbd0bdf5