Fix NPE in DialogLaunchAnimator

This CL fixes a NPE happening in DialogLaunchAnimator if it tried to
synchronize the 2 view roots at the start/end of the animation and that
the view that triggered the animation was detached from the hierarchy.
This regression was caused by the refactoring in http://ag/20139844.

Bug: 256194999
Test: Manual
Change-Id: Id264ddfa6365bfdc952d512c2c182b6cbf4906e4