Fix crash with app pairs and grid migration

This CL makes it so that app pair contents are loaded properly during grid migration, like folder contents.

Fixes: 316052056
Test: Have app pairs on home screen, change home grid (e.g. 4x5 to 5x5), launcher does not crash
Flag: ACONFIG com.android.wm.shell.enable_app_pairs DEVELOPMENT
Change-Id: I6a531d0d29d15ed7b8dfe7933690effe5344d941