Delete app pairs on model load if flag is turned off

This CL adds a case to processFolderOrAppPair() -- if the ENABLE_APP_PAIRS flag is turned off, we do not load app pairs into launcher.

This prevents a case where a user uses backup+restore to migrate an app pair onto an environment where app pairs is disabled.

Fixes: 330412376
Flag: ACONFIG com.android.wm.shell.enable_app_pairs TRUNKFOOD
Test: Manual
Change-Id: I4a97e4aadc620f8b727439fcdd70ab0e7a3005a8