Give an error/warning if a Xamarin.Android app references another app

[jonathanpeppers]

Steps to Reproduce

This scenario is currently only supported with Android Wear.

A case of a problem hit when doing this: #3342

Expected Behavior

You get a build error if you try to do this.

Actual Behavior

You are allowed to have an app reference app. You will probably encounter bugs/issues.

Version Information

Any