Remove stage concept from split screen APIs

Since split screen has transferred from stage-based model to app-pair
model. Update to remove the stage concept from split screen APIs, so the
caller won't need to deal with stage concept.

Bug: 250875492
Test: atest WMShellFlickerTests
Test: manual check start tasks or start intent/shortcut with task flow
Change-Id: I36bfb54f3f690cfafd18ba31ab23f5745ead6d97