Bet the Dreamcast threw some of these people for a loop.
It runs Windows!!!
Yeah Windows CE Kernel I believe.
But anyways.
To elaborate more on a previous post I made, and expanding on some of the best points made in this thread, borrowing a few choice statements and putting it together.
Android is just an operating system.
In regards to games and ports, Porting isn't that hard. However porting on a undocumented, not well supported and unoptimised API is. While choosing for android has little significance on porting itself, a big advantage is that a lot of the Android API's are very well documented. If Nintendo would've written their own OS and SDK, they also have to build a support desk for this and write the documentation for it.
http://www.develop-online.net/tools-and-tech/the-top-16-game-engines-for-2014/0192302
Android is covered in alot of these and most of them are the AAA dev engines too.
CryEngine, Unity, Unreal, Havok.
Not listed but have Android support Source 2 and a few others.
As for the ease of piracy on Android applications. There is no reason why they couldn't put a custom encryption/verification model on top of android and only allow encrypted/signed apps to run.
There are benefits to using an existing and flexible OS rather then reuse the Wii U OS for example. For one thing you'd have an unified account system that would tie itself into the Nintendo eShop meaning all of your purchase history would be detailed on the account level and not tied to THAT specific hardware like was done on the both the Wii, DS, and 3DS earlier on.
Nintendo basing their next OS on a mature, extensively documented and widely-supported architecture is certainly a very good thing.
And the biggest thing, It has little baring whatever hardware configuration used. The OS or Software kernel used does not dictate the level of power hardware has, that itself would be entirely dependant upon what CPU, GPU, RAM, etc being used. They could make a console or device at PS4 level of power or comparable to a mid to high gaming PC and still have the Android OS on top.
I'm looking overall at the bigger picture. And factoring all of the above it paints a better picture then what Nintendo is doing now with the Wii U OS / Nintendo Account thing.