Initial API for providing suggestions.

This is a WIP system api that will be
 - Implemented by one unbundled app (SettingsIntelligence) using system-sdk,
 - and consumed by Settings app.

Note: The bundled app does not have permission to read/write any
      user settings. Nor does it have permission to call any private API
      to do anything unsafe.

Test: builds
Test: instrumentation test
Bug: 65065268
Change-Id: Ib190c0e4c167deb3c6197b8a5b39d442d804770b