Fix NPE in LauncherSwipeHandlerV2Test.setup

RecentsDisplayModel doesn't need to be a mock. If it is mocked, then RecentsDisplayModel.getFallbackWindowInterface also needs to be mocked.

Flag: com.android.launcher3.enable_fallback_overview_in_window
Bug: 377678992
Test: pre/post submit
Change-Id: Iceb7c9cf6e9c3b0f3d08f85097ee3fde7e71f157