When putting pip into a split stage, evict previous tasks

* Currently previous tasks get evicted in
StageCoordinator#startPendingEnter, however that never
gets run for a Pip expanding to split because PipTransition
is the handler that gets run.
Fixes: 319114489
Test: Repro steps don't cause crash, did various tests around
entering and exiting split manually. Bugs exist, but not because
of this change
Flag: None

Change-Id: Ic61f4c839123f961495cf35d2ea8e83d988a607c