Reimplement PIP animation to not use drag resizing.

When using drag resizing it is difficult to keep big surface
surfaces (e.g. main app windows) and child windows in sync
as we resize. Furthermore it's difficult to resize child windows
quick enough to achieve more than a few frames a second as we have
to propagate through the client UI thread. Our new implementation
uses window scaling.

Bug: 26454664

Change-Id: Iac96619cefc075b1412cfeba3d3c9bcd7ce22f52