Show all TaskViews when doing seamless overview rotation

* If user is scrolling and rotates overview in fake
landscape, we wouldn't change the alpha to show if it
was hidden previously.
Now we only skip changing the alpha if it's already visible.

Bug: 194715506
Test: Scroll overview in fake landscape and rotate.
No more invisible task.

Change-Id: Idb519125fbb5ec7be33e0d7b6dd313fb4df684db