Allow flipping camera mode for front/back camera on Scan

Some phone models have both a front/back camera, so on the Scan screen we need to default to the back camera and add a button to let you toggle front/back camera being used for the scan as appropriate. Should also cache the result as a setting. It will USUALLY be the back camera, but it seems some phones default to front camera (which is annoying for users that don't want that), and perhaps some users prefer the front camera in some situations like if they are scanning off a cashstamp or something.