Fix keyboard navigation when answering drag-and-drop image questions

Discovered while testing MDL-58645. To replicate the problem:

1. Tab into a drop zone, hit right arrow or space to choose an item
2. Tab into the next drop zone and hit escape (not covered in testing, but works in weekly to unset the value)
3. Shift Tab back into the original zone containing an item and try to use right arrow. This is now stuck and won't respond to keyboard input.

Given this isn't the case in the recent roll (1afe68f38), it looks like something in MDL-58645.

We'll probably also need to check other related issues MDL-68113 and MDL-68382 to make sure those question types don't also need a fix.