Animate content fill animation to Recents Go

This CL adds the animation that occurs when transitioning from a set of
empty views to the actual task list after it is loaded. This is done by
setting a one-shot item animator that animates changes, for item views
that fade from empty to filled, and removes, for when we have too many
empty views.

Bug: 114136250
Test: Artificially increase task list load time and see animation fill
Change-Id: Ibbc09db702e591063ceea61df2359f18a3fcf8f9