Polish Enter pip transition from AE split.

Before: We were letting PipTransition animate one TaskFragment and no
one was animating the other one.
After: Separate Pip and AE fullscreen transition request, dispatch to
two Handlers.

Before: https://drive.google.com/file/d/1qch2kK8KYY7Yao1wg2TpKk9M1yDd4v4r/view?usp=sharing
After: https://drive.google.com/file/d/1M2Q2ZW03i9dWdQ4PYXW54jQP2_PimhBD/view?usp=sharing

Bug: 259671254

Test: atest

Change-Id: Ia9bf1e90ea23ca5f127b3effa1f63d2e68aac36c