Let default transition handler animate customized animation

Otherwise ActivityEmbeddingController will use default task
fragment animation, which may add edge extension on an translucent
style activity with x-translate. That looks weird.

Bug: 273434356
Test: Use ActivityOptions#makeCustomAnimation in an embedded
      activity to start next activity in the same task fragment.
      It should not show edge extension and default translate
      animation.

Change-Id: I9ab8cab6b9b46b4a358b2852dbbadd3a0e449aca