Hooks for interracting with stage split components using legacy recentsanimation

Recents is pretty complicated, so the best way to deal with this is to
have launcher notify/query shell when entering recents. This gives
shell a chance to prepare and then provide relevant leashes (for now
just the bar, but it may include others in the future like side-
outline).

Bug: 192292305
Test: Enable SPLIT_SELECT and go to recents from split.
Change-Id: Iff6b14a1b0b95f207bdaa341c62fc6582659e923