Refactor storage & notification flow for prod

- Update storage to avoid mixing duplicate shortcut IDs from different
apps/users. The storage change unblocks calling the newly added
PeopleManager#getConversation API to directly retrieve the tile data.
- Update the notification flow for the newly added storage to only
update when there's an associated tile to avoid wasteful updates
- Defaults PeopleSpaceWidgetManagerTest to single conversation People
Tiles

Test: PeopleSpaceWidgetManagerTest
Bug: 178792356
Change-Id: I697401de700924a2ab5ed977dcce157deefd56b9