Rewrite xamarin-android build system with testable framework

[atsushieno]

Since we started using MSBuild as the project build script, this kind of build regressions #1491 (comment) have been occuring too many times, making it unreliable, unreadable, and slow. We need build script written in human-hackable language and make it open to everyone, not just one or two people who understands the spaghetti.

[jpobst]

This seems unlikely to change.