Allow non-resizable apps in split-screen (7/n)

Add interface for SizeCompatUIController in WM shell to use
when refactoring SizeCompatModeActivityController from system UI.

Bug: 176061101
Bug: 178327644
Test: atest WMShellUnitTests:ShellTaskOrganizerTests
Change-Id: I37f72396e1769b7773722be6575506fc430b2739