Ensure BiometricDialogView dismiss animation is run

postOnAnimation requests the animation be started on the next frame. The
behavior is undefined if an animation is already running. Currently
if this happens, the second animation is never initiated.

Fixes: 138199986

Test: Modify BiometricPromptDemo to cancel authentication immediately after
      requesting authentication. BiometricDialogView isn't stuck anymore

Change-Id: Ia62dccc2d484d1eaca5587bffa4d3def89730533