Map shell transition to recents animation

This hooks up the essential start/finish. One thing to note
is that it uses the normal startActivity rather than a special
startRecents call for shell transitions.

Bug: 162503077
Test: Enable shell transit. Open an app, use gesture to open
      recents.
Change-Id: Ia51fa8baeb43bf3fa768b25a840b629ec94e16a1