I don't have a 7.1+ version of Android, but don't apps like Netflix and other video players inhibit those soft buttons to take advantage of full screen real estate? If so, then iFly could do that, too, if it wanted to.
But one of the benefits of Android is the ease with which you can have multiple apps running and switch between them quickly. In flight, I often switch between iFly and other apps, like a PDF viewer to look at performance charts, FltPlan GO for my flight plan and my destination airport FBO info, MyFlightbook to record flight log data, etc. I'm not sure what gesture is required to get those soft buttons to appear, but if it's as simple as tapping the screen, then you're still going to have the problem we're talking about because they'll be there any time you're interfacing with iFly, and if it's more complicated that tapping the screen, it might become annoying to users who flip among multiple apps often.
Which means it might need to be a user preference, which would come with its own set of concerns (another user preference, an Android-unique behavior, etc.)