Don't dispatch onBackCancelled if start is not dispatched.

Bug: 329491664
Fixes: 329491664
Test: long press on the edge of all-apps (built from SysUI-Studio). Make
sure there's no crashes.
Test: atest BackAnimationControllerTest

Change-Id: I0d28373b3341e6637dfaf0adaf7b3ddf0196f69b