Remove stage type from split screen APIs

Due to stage type will soon be deprecated, updates to remove stage type
param from split screen APIs and leave resolving stage job to split
screen controller.

Bug: 198438631
Test: atest WMShellUnitTests
Test: manul check drop to split and shell command to split behavior
Change-Id: Ic543d74ebbee4afba06187e382808fa75ad8a416