Fix NPE in LauncherSwipeHandlerV2Test.setup (follow up)

Since there is no display being used for LauncherSwipeHandlerV2Test, then no RecentsDisplayResource gets created for the DEFAULT_DISPLAY (and so RecentsDisplayModel.getFallbackWindowInterface returns null). Adding a mock display

Flag: com.android.launcher3.enable_fallback_overview_in_window
Bug: 377678992
Test: pre/postsubmit
Change-Id: I43754eafb4f6a9b393e6a7b7fbd5b27903d2f9b9