Allows the system window to be shown during the PB animation...

...if the predictive back animation triggers a fixed rotation animation.
So if the edge back window is drawn after the async rotation animation
has started, it won't remain hidden.

Flag: EXEMPT bugfix
Bug: 368296326
Test: launch app then rotation device, trigger back-to-home. Verify
the edge back window can be visible after gesture started.

Change-Id: Ib3f52357ea02a929348c97c86577eedb53243f47