Reparent folders and app pairs

Previously, app pairs and folders shared a common data model, FolderInfo. Now we need to separate them, so a new type, CollectionInfo, will serve as the parent of both types.

Bug: 315731527
Fixes: 326664798
Flag: ACONFIG com.android.wm.shell.enable_app_pairs TRUNKFOOD
Test: Manual, unit tests to follow
Change-Id: Ia8c429cf6e6a376f2554ae1866549ef0bcab2a22