Finish converting tile usage to task-org

Right now, there are situations where WM and task-org are fighting
over surface positioning. Most notably this was visible when starting
the first minimized task.

This finishes making the tiles (root tasks) be de-facto task-org'd
tasks. This improves a lot of glitchiness in split-screen.

Bug: 149952263
Bug: 150954849
Test: launch an app into split-screen primary and notice that it
      glitches less.
Change-Id: I727da6cf6c9b887cb698fe1b764f3835d566dec1