Keep it fullscreen when fullscreen app relaunches

This CL prevents the desktop-first policy from forcefully changing the
windowing mode of the fullscreen app if it’s a relaunch of the existing
visible fullscreen app.

Flag: com.android.window.flags.enable_desktop_first_fullscreen_refocus_bugfix
Bug: 417908426
Bug: 419621913
Test: DesktopTasksControllerTest#handleRequest_fullscreenTaskRelaunch_returnNull
Change-Id: I5a6b12f44dda0371f4be954fcdf986e06abae0e7