Add a new class to make split display aware

Add a helper class and a data class for managing and storing split
screen related tasks and stages. This will allow StageCoordinator to be
display aware in the follow up CLs.

Flag: com.android.window.flags.enable_multi_display_split

Test: manual
Bug: 393217881

Change-Id: I2ab78c4dd0525621a0db96ea9e147c5fa60578aa