Allow app pairs in folders

This CL substantially refactors folders to be able to take contents of type AppPairInfo. App pairs can now be moved in and out of folders, and launch from folders.

This CL contains only logic and model changes; animation and style changes (for dropping items into folders, color changes to app pair surfaces, etc.) will be in a following CL. Another CL (hopefully) will contain tests. I'm planning to submit them together, but this patch should also be able to stand alone with no issues (except janky transitions).

Bug: 315731527
Flag: ACONFIG com.android.wm.shell.enable_app_pairs TRUNKFOOD
Test: Manual, more to follow in another CL.
Change-Id: I73732fcaefbdc61bf6e02a5be365962b8bbc3e41