Google's Android mobile operating system is open source, which means anyone can, in theory, build their own mobile operating system based on the Android Open Source Project (AOSP). Why, then, are there not many different versions of Android to choose from?Well, the good news is that there are a few alternatives to Android—some of them are excellent if not better than the version of Android that ships with your smartphone. But the reason there are only three or four good options highlights the fa...