Don't draw the task graph line image on devices by default

Summary:
See downstream https://phabricator.wikimedia.org/T210482.

On mobile, the task graph can take up most of the screen. Hide it on devices. Keep it on the standalone view if you're really dedicated and willing to rotate your phone or whatever to see the lines.

Test Plan: Dragged window real narrow, saw graph hide.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20313