Adapt transition merge during recents into onTaskAppeared.

If an incoming transition merge request contains only a task
open, this will be converted into the existing onTaskAppeared
callback that recents expects.

Bug: 186132704
Test: open app, swipe to overview, scroll to another app, launch.
Change-Id: Ifd288fbb73e2b4ea7011066bda5c3694b5f579d9