Fix wallpaper exposure bugs.

Qualify the test for wallpaper animation to exclude the dummy
animation. This keeps us from treating a dummy-animating wallpaper
as an exiting wallpaper and providing the wrong animation.

Hide wallpapers when the wallpaper target window is hidden. This
fixes a timing issue where the wallpaper was exposed for one pass
through performLayout after the launcher was hidden.

Fixes bug 6454992.

Change-Id: Ib4f9205c01a37e6f48f1f93ddcf2476e40ff942f