Remove supportsSplitScreenMultiWindow

For new split screen, we are using supportsMultiWindow and other
multi-window features should use it too so remove
supportsSplitScreenMultiWindow and update some files which still
use it.

Bug: 199236198
Test: pass existing tests
Change-Id: I3b3ec3376a4c79295f9d9f913585534a3815e75a