Don't try to load a long screenshot image more than once

If the preview already has a drawable onStart, there's nothing else to do.

Previously we'd always try to load an image onStart which would break if
onStart wasn't caused by either an initial load or a state restoration.

Bug: 193045988
Test: When cropping long screenshot, swipe up to overview, then return.
      Previously, the activity would disappear, now it continues undisturbed.

Change-Id: Ief1f10ce621e7f25bf3acbcf04ad3b6c04a28577