Fix dialog exit animation

This CL fixes a bug that could happen when animating a dialog out at the
same time as hiding the SystemUI shade, and that would make the phone
unusable until the shade is swiped down again. See b/223387276 for more
info.

Bug: 223387276
Test: Manual, see b/223387276#comment1
Change-Id: If251b00558a5ca9a927d1be2bb015f1c0acb2d57