Migrate recents.splitPrimaryTask to WM Shell library

There were some legacy ways to trigger split screen through
Recents.splitPrimaryTask. Decouples the API from Recents and migrates it
to WM Shell library.

Removes SplitScreen.onDockedTopTask and SplitScreen.onRecentsDrawn
because we have SplitScreenTaskOrg to instantiate new deivider bar and
resize primary/secondary split after entering split screen mode.

Fix: 169056724
Test: atest SystemUITests
Test: atest WMShellUnitTests
Test: manual trigger splitPrimaryTask api and check
Change-Id: I68979e2e32c7ff36d35ae451c81dcf09e2dc5ef6