Migrate splitscreen to WM shell lib (1/n)

To prepare for WM shell lib migration, extracts APIs from divider to
decouple dependency between systemui and divider logic. The Divider as
one of the SystemUI component holds DividerController and is in charge
of interacting with WM shell lib.

Bug: 161116823
Test: atest SystemUITests
Test: manual verified divider feature
Change-Id: I7e97599c369dd38b4b7da82fef53ab4de240f251