Deflake testCancel_sneakyCancelBeforeUpdate

Waiting one frame for the animation to start is a safe bet. Waiting an
additional frame for the cancellation to take effect isn't.

This test was 50% flaky before.

Test: atest --iterations 10 'com.android.server.wm.SurfaceAnimationRunnerTest#testCancel_sneakyCancelBeforeUpdate'
Bug: 71719744
Change-Id: I97ab43e0a31107bd0497a8e02be88cf331d71ce1