Only allow main window of recents target become the wp target

...otherwise an overlay window might be treated as wallpaper
window, which causes a show -> hide -> show wallpaper infinite
loop, leading to these layout repeats, causing issues.

Test: Swipe up
Fixes: 134986295
Change-Id: Ic44c40a69414dbe6c35f63f45cf3a17682ad0e85