[People Service] Remove conversations without corresponding shortcuts in ShortcutManager

In certain exceptional circumstances, shortcuts might be removed from `ShortcutManager` without the People Service being alerted. In these case, we remove the corresponding conversation during the `getConversation()` path. This will clean up these orphan conversations to ensure that they don't take any space or memory.

Bug: 243632873
Change-Id: I798b93a7fdea556cb3b4e03a87cbfe5240d745ee