Fix ActivityLaunchAnimatorTest crash

This CL fixes a crash in a test that was introduced by a last minute
change in http://ag/15235792. This crash was not caught by the
presubmits for some reason.

Bug: 192194319
Test: atest ActivityLaunchAnimatorTest
Change-Id: I082514703a2c5500686a6bfe3cec3501653032b3