For reference, and, perhaps, discussion: 'Android' means lots of different things, and there's a lot of confusion about forks, Xiaomi, China and AOSP, as well as 'the next billion'. So this is how I try to think about this. First, there are actually (at least) six types of 'Android' in the market today:
'Stock' Android, as seen on Google's Nexus devices, complete with Google services (but with tiny unit sales)
'Modified' Android, as seen on phones from Samsung, Sony, LG etc, complete with Google services - generally, these are modifications that no-one especially likes, but which Google explicitly allows
'AOSP' or open Android, as seen in China - essentially these phones are the same as number 2, but with no Google services and apps from the Chinese portals embedded instead. Hence Samsung, Sony etc sell their phones in China without Google services, but few other changes
(or perhaps 3.1) 'Modified' Android as seen on Xiaomi phones and those of its followers, which people actually seek out, and which comes without Google services in China and with them elsewhere
ROMs and third-party implementations of Android that are available for any handset, such as both Xiaomi's MIUI and Cyanogen (an a16z portfolio company), which may or may not have Google services included or accessible. Again, these contain optimisations and improvements that make people seek them out
Forked Android, such as the Kindle Fire phone: Android heavily modified to produce a different experience, and Google refuses to allow Google services to run on them (other than plain old web search, AKA POWS). Note that Xiaomi and Cyanogen are not forks.
Th first two or perhaps three I would describe as 'closed' Android and the second three are 'open' Android, certainly from the perspective of device manufacturers. The first two (actually just number 2) have over a billion users outside China (as of the numbers given at IO last summer). Versions 3 and 4 have a further 400-500m users, almost all in China, and there are perhaps 50m users of 5 ( a very rough estimate) both inside and outside China, partly overlapping with the others. Six - well, ask Amazon.
In parallel, it's worth breaking down Android users in a similar way:
ROM users (very roughly, perhaps 50m people)
People who like to install the kinds of apps that do things Apple doesn't allow on iOS and Google does allow on Android (note that Apple now allows rather more things and Google does not, oddly, allow gambling apps). I had a go at quantifying this here
People with a personal preference for Android, who none-the-less do not actually install ROMs or do many things that are blocked on iOS (the difference between this and 2 is a grey area, obviously)
People who don't actually care very much one way or the other between Android and iOS, and (for example) got a good deal, preferred the handset design or (especially) the larger screen size that used only to be available on Android, and indeed might switch back and forth between iOS and Android
People who can't afford iPhones or other high-end phones and so got Android as the cheaper option.
People who actually don't care about smartphones at all, and so just bought a 'cheap phone' (or just a phone with a good camera, say), and happened to get an Android since it's taken over most of the mid range and low-end, and who don't do much with the ecosystem
People in emerging markets who really can't afford anything other than a $50 or $100 Android phone but are enthusiastically taking advantage of everything it can do.
As above, but have a relatively expensive data plan, limited 3G coverage and, often, limited access to power to charge their phone (this one is is where the 'next billion' will sit)
Some of these categories (but obviously not all) also apply to iOS, of course, but selling phones only at $600 for the latest model creates a more uniform customer base.
Layered across both of these is huge geographic variation. The must-have phone for teenagers in San Francisco and Jakarta is very different. But the underlying point about both lists is that tech and mobile have grown far past the point that there is really a single market for anything. When you connect everybody you get, well, everybody, and they're not all like you.