Wallpaper target selection during animation

In case of double tapping from Home, there could be two set of
transitions, first is launcher->Recents, followed shortly by
Recents->app. On the second transition, it should be running
a wallpaper close, but because of the way we pick wallpaper
target, sometimes we pick launcher if both launcher & Recents
are animating away. This makes us to run a different animation
which could involve dimming the wallpaper briefly.

Also, findWallpaperTarget() sometimes could toggle between two
valid wallpaper targets, picking either of them gives correct
result for showing the wallpaper, but could result in different
exit transition.

This change prefers the visible target if there is only one
visible, otherwise the one in opening or closing app list. This
helps maybeUpdateTransitToWallpaper to better choose transition.

bug: 30831873
Change-Id: I0eaa89bdc35f5f51875d5cbeceba11ce40f4791f