[generator] Ensure non-constant static interface fields are generated as interface properties.

[jpobst]

Fixes: #821
Context: dotnet/android#5818
Context: #459
Context: #509

When we began generating static methods and properties on interfaces when running with DIM enabled, it looks like we did not include static fields that cannot be treated as constants.

Thus, given the following example from dotnet/android#5818:

public interface EGL10 {
  public static final int EGL_ALPHA_FORMAT = 12424;
  public static final EGLSurface EGL_NO_SURFACE;
}

we only generate:

public partial interface IEGL10 {
  public const int EglAlphaFormat = (int) 12424;
}

This is an issue because we [Obsolete]'d the version in the *class* EGL10 pointing to the non-existent interface version:

[Obsolete ("Use 'Javax.Microedition.Khronos.Egl.IEGL10.EglNoSurface'. This class will be removed in a future release.")]
public static Javax.Microedition.Khronos.Egl.EGLSurface? EglNoSurface {
  ...
}

This commit generates the static interfaces fields as properties when -lang-features=default-interface-methods is specified.

[jonpryor]

Shouldn't this interface declaration also contain:

public const int EglNativeVisualId = (int) 12334;

I would expect the IMyInterface type to "mirror"/contain the same members as the MyInterface type, yet IMyInterface.EglNativeVisualId doesn't exist.

[jpobst]

Interface constants are gated behind -lang-features=interface-constants, which isn't enabled in this test suite.

I went back and forth on this and decided that "static interface fields" falls more under default-interface-methods than interface-constants. DIM is poorly named and really means "default AND static interface members" in this context.