I've played around with this a bit, and I don't believe there is a good way to do it.
The best and obvious solution would be to use Layer Styles to add a stroke to the layer, but that's not available in PS for Mobile. My next thought was to "stroke selection," but that isn't available either. Creating a shape layer that had a separate fill and stroke is another possibility, but PS for Mobile doesn't have shape layers.
The only thing I could do, and it's really really a stretch - is to create a rectangle or square selection, and fill it with black. Then, in the selection options at the bottom, choose the 3 dots and "transform selection." Manually move the corners in a tiny bit to make a smaller shape. Then, fill the smaller shape with white. Voila - you have a white rectangle with a black border. If you're careful, the sides might even look like they are the same width.
As a follow up, I found a much better workaround. Use Adobe Comp for iPad to set up your document, lay out the frames, adjust the size and stroke width. Then, share the document as an image to your camera roll from Comp, and import that image into Photoshop. Much less hassle.