[tests] use a "real" Xamarin.Forms app

[jonathanpeppers]

In the past XamarinFormsAndroidApplicationProject was incomplete:

1. It was not a "real app", but the resemblance of one that merely
   compiled.
2. It did not have XAML or import/run Xamarin.Forms MSBuild tasks.
3. It was not an app that could startup, and render Xamarin.Forms
   content.

So I took the simplest Xamarin.Forms template from Visual Studio 2017
15.8.7, a Blank App:

• Added various new files as EmbeddedResource in
  Xamarin.ProjectTools.
• Used most files as-is, adding ${ROOT_NAMESPACE} where appropriate.
• Included Tabbar.axml, which has a "custom view".
• Included proper XAML files.
• Imported Xamarin.Forms.targets, and verified that XamlG and
  XamlC are now both running.
• Added Xamarin.Android.Support.Vector.Drawable, which avoids a
  crash at runtime.
• Updated to Xamarin.Forms 3.1.0.

Overall, this should make any of our test cases using
XamarinFormsAndroidApplicationProject much closer to reality.

[jonathanpeppers]

Whoops, something broke

[jonathanpeppers]

This is good now, I had a silly mistake.