Fade out the taskbar icons when animating to overview in 3 button nav.

Bug: 231213105
Test: from home, opened recents, then went back home, then went back to recents, resumed an app, then went back to recents and resumed the same app (both in 3-button and gesture nav)

Change-Id: I83b1b33cafb95353f36a126b93fcaf3b536d3bf4