Keep track of TextEditor focus in Alert transitions.

When transitioning from a Canvas Screen to an Alert Screen, keep track
of which TextEditor has focus. When returning to the Canvas Screen,
focus the TextEditor. This helps us match app expectations with regard
to keyboard visibility.
Nit: I would make *compareScreenToReferenceImage* do the threshold check, which'll come in handy once we add more such tests; but we can do that in a followup.