Adds back gesture recognition to Sandbox.

This is used to step through the "happy path" of the existing
back tutorial steps (right edge and left edge). Unlike in the
Pixel Tips app, the tutorial only continues if you perform the
gesture from the correct edge of the screen.

This also lays the groundwork to provide helpful tips if the
expected gesture is performed incorrectly, although currently
such gestures are just ignored.

Demo: https://drive.google.com/open?id=12S42mZQITGzIWnj7mP1L__PCOgAQcjgp

Bug: 148542211
Change-Id: Ib2e0b2ff7c021db48c96d58e1370fa2e93330912