add the ability to adjust the screen size / aspect ratio

[devoncarew]

Likely exposed in the flutter view (and similar to the ability to toggle the platform) we should add the ability to easily explore what different screen sizes and aspect ratios would look like.

[eseidelGoogle]

See also flutter/flutter#5026. Flutter has all the tools to make this possible. For example when you run a flutter test on the device, we force the aspect ratio to match the test, not the phone hardware.