Fix unbounded number of uncached shortcuts

There is a consistency issue in which some cached conversations in
People Service do not get cleaned up because they are missing a event
timestamp. For those cases, we use the creation timestamp to order by
recency and still clean up those conversations.

This change will enforce that at most `MAX_CACHED_RECENT_SHORTCUTS` are
cached by People Service for uncustomized conversations as expected.

Test: Manually with app that creates cached shortcuts and unit tests
Bug: 233926836
Change-Id: Ia017917ea56f2e233fdb837e574a8bd7c0d69e3b