Migrate splitscreen to WM shell lib (4/n)

Moves systemui.stackdivider package to wm.shell.splitscreen package.

Bug: 161116823
Test: atest SystemUITests
Test: atest SystemUIGoogleTests
Test: atest CarSystemUITests
Test: manual verified split screen behavior
Change-Id: Ib62450b78da454a7dc93a88272c87b8c12fcc1d8
54 files changed