Migrate splitscreen to WM shell lib (3/n)

Extracts interface of split screen feature to expose APIs for
SysUISingleton scope to use.

Bug: 161116823
Test: atest SystemUITests
Test: atest SystemUIGoogleTests
Test: atest CarSystemUITests
Test: manual verified split screen behavior
Change-Id: Ic5ace86a2205cde53c074d1c0b0bf8a40e0a2968
21 files changed