[PiP2] Handle split to PiP as a mixed transition

Make sure Split-to-PiP case (w/o replacement) is handled
by the DefaultMixedHandler, and starts a dismiss split transition
as a follow-up when dispatching the transition for the leftover handler.

Also make sure we delegate the PiP part of the animation in PiP2
similar to how it's done for TYPE_OPTIONS_REMOTE_AND_PIP_OR_DESKTOP_CHANGE
or TYPE_ENTER_PIP_FROM_ACTIVITY_EMBEDDING.

This fixes the single-activity PiP case where split is dismissed;
multi-activity case is yet to be refactored fully.

This is only handling the swipe-up from split case, btn-nav case is yet
to be fixed.

Bug: 381332669
Flag: com.android.wm.shell.enable_pip2
Test: manually repro the steps in the bug

Change-Id: I062db7850d94e59e906bb5fe72f50da30e0dd260