Prevent pinned stack from having extra elevation

Pinned stack doesn't have focus, so there's no need for setting extra
elevation to show shadow for focused case.

This removes extra eleavtion for pinned stack, and improves the PIP
animation quality by preventing extra surface size change
at the end of animation.

Bug: 27364161
Change-Id: Id099a78de48b2e038a69600c94454b5cbfe0628f