Cleanup the legacy split screen (2/n)

Remove WINDOWING_MODE_SPLIT_SCREEN_PRIMARY and SECONDARY used on
tests and replace them by WINDOWING_MODE_MULTI_WINDOW or other
migrate way.

Bug: 199236198
Test: atest WmTests
Test: atest DragAndDropPolicyTest
Change-Id: I41b25392d710eeea42edf75b3c9bca20a7d8a887
11 files changed