This would have the potential to be sooo awesome... If the Pi3 actually came with more then just the 1GB of LPDDR2, that came with it. As it is... Android Marshmallow (Or Tedy Nuget McNugatface... When it lands in the Fall), will likely run like a two legged dog, with a bad limp.
Other then that, I'd be all over it, as a replacement for my chepo Chinese Android MXIII Box which will never see an update past v4.4.2. And, due to it being Chinese... Sources with which to build something like CyanogenMod 12.1 / 13.0, are virtually impossible to get a hold of.
That my new masochistic hobby is in fact building Android from source I'm sure I could have endless hours knocking my head into the Keyboard with that... But, again whats the point of it, when you just don't have the RAM to comfortably run it?