Support to merge transition to remote handler while in split screen mode

Support to merge transition to activating remote transition while in
split screen. Like the case openning a normal app from overview during
split screen mode, it'll trigger a new app openning transition which
should be merged back to the remote transition running in launcher side
to finish the recent animation.

If a split dismissing transition was triggered right after recent
transition finished. It might be a no-op transition and thus won't
callback startAnimation. So also reset split screen state and visibility
when finishing transition.

Bug: 206487881
Test: enable shell transition and enter split screen, observed the
      transition switching to normal app won't hang.
Change-Id: I9bac5eab844f7835c69f9aad17a5c50dc01a76fa