Only allow focused window to select seamless rotation.

We require apps to opt in to seamless rotation. If a dialog or some such
has popped up above them, the dialog may not be prepared to rotate
seamlessly and we should just fall back to the old behavior for now.

Bug: 30968345
Change-Id: Ib9a7352ed84d41076bdd162bf38d151b053a307e
(cherry picked from commit 828ec3dc)