Launch app pair when one of them is in freeform.

Call launchAppPair when one of the tasks is in freeform to prevent only
launching one app to the side and choosing the DesktopWallpaperActivity
as the other app pair.

Fixes: 394917143
Flag: com.android.window.flags.enable_desktop_windowing_mode
Test: atest NexusLauncherRoboTests
Change-Id: I3967473fa648f2b245ac8b4f16e6112f6d8f077f