AndroidRuntimeSettings.h is under Epic Games folder, not a file I created nor modified. Looking at the header file, the case is already correct (bFullScreen). Also I have nothing to do with Android. I compile for Win32 and Win64.x
I had the same issue migrating from 4.6.1 to 4.9.1, though it was pointing at a different file (BTTask_RotateToFaceBBEntry.h) and value (precision). Turns out it was one of my plugins, which had “precision” as a parameter. I removed all my plugins and was able to build it, so I started adding them back until it threw the error again. Might be worth looking for instances of bFullscreen in your own code, and change them to see if it quits.