Remove screenshot animated view

Simplifies the layout/animation code of the screenshots UI by
removing the "screenshot_animated_view" and instead just
animating the preview from the starting point to its static
location. This change supports the swipe-dismiss change by
making the underlying views easier to work with.

Bug: 157223526
Test: manual; tested from keychord and overview to ensure no
visual change

Change-Id: I52525aaa0743c5b15f515a50aa79f8dfb547fdff