Ask remote transition handler to handle transiton in MixedHandler

When DefaultMixedHandler#handleRequest is going to handle a remote
transition, also ask remote transition handler to handle the remote
transition, so if the leftover handler cannot handle the transition,
the remote transition handler can have chance to handle it.

Bug: 301047305
Test: create a split screen pair, then launch app from notification,
verify remote transition handler can play the transition while split
cannot handle it.
Test: create a split screen pair with one of app is pip-able, then
launch app from notification, verify the app can enter pip, and the
remote transition handle can play the open transition animation.

Change-Id: I84a4b7c26c57498d17baca58811db19f00aa2714