(In case it matters, these are Java Android projects)
Project A is a library project.
Project B depends on A
Since this is specifically for Android, have you looked into Android libraries?
In my case I have a free (B) and a paid for (C) version of my app. Both share a common code base (A). In eclipse I make A an Android library and both B and C use this library and all 3 are separate git repos. With this setup, changes I make in A are automatically available in B and C and I only need to maintain one code base for each project. No fancy build scripts/configs.
I think this sort of simplicity is what you were after? Apologies if I have misunderstood the question.