Fade recents thumbnail to transparent earlier.

Reduce the gpu load by fading the recents thumbnail to an alpha of
0.0 before the remaining animations are completed. When alpha hits
0 the gpu treats the layer as hidden and can merge the remaining
layers in time.

This is a partial fix for 7729214.

Change-Id: I9761bbd0554db6454c7eec0485be798b11672ff5