Simplify finding wallpapers

Because mWallpaperTokens already contains all wallpaper windows,
it is unnecessary to traverse all windows. Then it can be 10x faster.

By making mWallpaperTokens follow z-order, that also improves the
correctness of getTopVisibleWallpaper.

Remove startWallpaperAnimation by the way as it is dead code for years.

Bug: 163976519
Test: WallpaperControllerTests#testShowWhenLockedWallpaperTarget

Change-Id: Ifa3bf54ebeee049ccc1952884d4a213a2ee9f39d