Implement the get/remove recent conversations APIs in PeopleService

A conversation is a cached recent conversation if all below conditions
are met:
1) its shortcut is cached
2) its notification setting is not customized
3) it has recent notifications

A recent conversation can only be removed if it does not have any
active notifications.

Bug: 162593584
Test: atest + manual test
Change-Id: I0d820a6a2bb82ceb238c3f559e108829d1547291