[.NET 5] use $(OutputType) Exe for application vs library

[jonathanpeppers]

In .NET 5, Xamarin.iOS is already using <OutputType>Exe<OutputType>
to distinguish an iOS application vs an iOS class library.

Xamarin.Android, however, is checking the existence of
Properties/AndroidManifest.xml.

This brings up some issues:

• When using a multi-targeted project in .NET, we should be using the
  same convention to distinguish library vs applications on both iOS
  and Android.
• Xamarin.iOS apps have a static void Main method. OutputType=Exe
  makes sense, as it tells Roslyn to expect a Main method.

To consolidate this, I propose:

• Use $(OutputType) of Exe to set $(AndroidApplication) to
  true. No longer look for AndroidManifest.xml for this default.
• Immediately set $(OutputType) to Library, since we never want a
  Main method on Android.

To make this work, I had to move these defaults from
Xamarin.Android.Sdk.props to Xamarin.Android.Sdk.targets. This
allows these properties to be evaluated after the values set in the
user's project.

I added a test that builds a library, checking that the
__AndroidLibraryProjects__.zip EmbeddedResource has the correct
contents.