Save multiple RunningTaskInfo in GestureState

* With new split screen running two active tasks for
a given gesture, we need to get all running task
infos instead of the single most recent used one
* TODO(b/210903248) for proper refactoring for
GestureState

Fixes: 205675364
Test: Swiping up on overview no longer crashes
Change-Id: Iea1f193149657182311a97f46007b4d43e8ad549