Polish app => recents animation (2/N)

This CL moves the core "app to overview" animation logic into
the main view as the newer animation would need to animate over several
of the main view's children and so the animation logic needs knowledge
of those children.

Bug: 132112131
Test: Build and test, functions as before
Change-Id: I9cbde55a582bee62e0a97e38c5fdf1d5841502db