Basic support for split-screen in new transition system

Sends the "trigger" taskinfo associated with a transition when
applicable. This can be used by the transitionplayer to make
decisions about how to start and play transitions.

Additionally, TaskInfo is provided in each Change object if
they are tasks. This provides all the information needed to
choose how to handle split.

Shell's transitionplayer makes use of this to add support for
"handlers" to deal with specific transition requests. Currently
a single handler is added to process split-screen transitions.

Bug: 169035082
Test: Use split-screen (enter, exit via swipe up/down,
      exit via dismiss top)
Change-Id: I53a03dd85cb260505e66d5d87a34afa4ab1f8cc8