In your case agile would help to avoid the eventual trouble when the client brings a phone for you, and you notice it's a Windows Phone instead of Android, and graphics need to be done in Direct3D, not OpenGL. That would not be unheard of. Having no client solves that problem, though.
Statement of work: Build engine for Android architecture
Sales Order: Pay me money net-30
Where's the confusion? If the customer provides a Windows phone, too fucking bad they signed a goddamned contract that explicitly stated what they needed and that was produced and delivered. Problem? See you in court. -Signed, Every corporation I've ever worked for