[generator] Don't register invariant+excluded types

[jonpryor]

Context: dotnet/android#732

Commit d7dfa0b broke the
xamarin-android/tests/CodeGen-Binding/Xamarin.Android.McwGen-Tests
build, as it started treating Android.Graphics.Color as a normal
type, instead of the special-cased behavior it should have:

error CS0452: The type 'Color' must be a reference type in order to use it as parameter 'T' in the generic type or method 'Object.GetObject<T>(IntPtr, JniHandleOwnership)'
error CS1503: Argument 1: cannot convert from 'Android.Graphics.Color' to 'Android.Runtime.IJavaObject'

The problem is that this code pattern:

static int n_UseColors_I (IntPtr jnienv, IntPtr native__this, int native_p0)
{
	global::Com.Xamarin.Android.Bxc4288 __this = global::Java.Lang.Object.GetObject<global::Com.Xamarin.Android.Bxc4288> (jnienv, native__this, JniHandleOwnership.DoNotTransfer);
	global::Android.Graphics.Color p0 = new global::Android.Graphics.Color (native_p0);
	return __this.UseColors (p0).ToArgb ();
}

became:

static IntPtr n_UseColors_LAndroid_Graphics_Color_ (IntPtr jnienv, IntPtr native__this, IntPtr native_p0)
{
	global::Com.Xamarin.Android.Bxc4288 __this = global::Java.Lang.Object.GetObject<global::Com.Xamarin.Android.Bxc4288> (jnienv, native__this, JniHandleOwnership.DoNotTransfer);
	global::Android.Graphics.Color p0 = global::Java.Lang.Object.GetObject<global::Android.Graphics.Color> (native_p0, JniHandleOwnership.DoNotTransfer);
	IntPtr __ret = JNIEnv.ToLocalJniHandle (__this.UseColors (p0));
	return __ret;
}

The cause reason for this is that d7dfa0b updated
SymbolTable.AddType() so that it no longer called
SymbolTable.ShouldAddType(), which is what would prevent
type registration of Android.Graphics.Color.

Reintroduce the call to ShouldAddType() so that
Android.Graphics.Color isn't registered.