Prepare StageCoordinator for 2x1 split-screen (1/n)

Refactor StageCoordinator to support the new 2x1 split-screen.
- Create a StageCoordinatorAbstract class to define the StageCoordinator interface.
- Create a placeholder StageCoordinator2 class for the new implementation.

Bug: 439992109
Flag: com.android.wm.shell.enable_2x1_split
Test: Ensure no regression for existing Flex Split-Screen.
Change-Id: Ib7574d5e242c9fba029dd42531a6627d01b83631