You do not want to run Android on the Raspberry PI.
Android uses a specialized linux kernel to allow Google to run its Dalvik virtual machine. Android is less about gnu/linux and more about Java... or their version of it.
Android is also specialized for a specific kind of hardware (phones and tablets). Android development requires a specific set of buttons, sensors, and video hardware to function correctly. Raspberry PI does not have the correct hardware configuration to correctly support Android. That is NOT to say that it couldn't be hacked to make it work, but man, what a headache. NO, don't even think about it.
If you want to run Android, get yourself a good Samsung tablet or Galaxy phone and enjoy (I love mine!).
The OS for Raspberry PI is Raspbian gnu/linux. (there are others, but I just gave you the best)