[linker] improve method detection in FixAbstractMethodsStep

The forms team run into an issue with debugger in VS [1]. During
investigation it turned out that our FixAbstractMethodsStep injects 2
methods, which were not needed, as the methods were already
implemented. The bug itself looks like an issue, where original mdb
file was used in the apk, instead of the one written by our linker.

This change improves the method detection, where explicit interface
methods name starts with `global::` [2]. In this particular case it
were these methods:

    global::Android.Views.View.IOnTouchListener.OnTouch
    global::Android.Views.View.IOnClickListener.OnClick

This is possibly compiler specific, so we rather look in
MethodDefinition's Overrides to find the interface method and compare
it to the interface method we are looking for.

[1] https://bugzilla.xamarin.com/show_bug.cgi?id=59293
[2] excerpt from ikdasm output:
    .method private hidebysig newslot virtual final
            instance void  'global::Android.Views.View.IOnClickListener.OnClick'(class [Mono.Android]Android.Views.View v) cil managed
    {
      .override [Mono.Android]Android.Views.View/IOnClickListener::OnClick

Author: radekdoulik

src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/FixAbstractMethodsStep.cs

@@ -95,8 +95,23 @@ static bool CompareTypes (TypeReference iType, TypeReference tType)
 			return true;
 		}
 
+		bool IsInOverrides (MethodDefinition iMethod, MethodDefinition tMethod)
+		{
+			if (!tMethod.HasOverrides)
+				return false;
+
+			foreach (var o in tMethod.Overrides)
+				if (o != null && iMethod == o.Resolve ())
+					return true;
+
+			return false;
+		}
+
 		bool HaveSameSignature (TypeReference iface, MethodDefinition iMethod, MethodDefinition tMethod)
 		{
+			if (IsInOverrides (iMethod, tMethod))
+				return true;
+
 			if (iMethod.Name != tMethod.Name && (iMethod.DeclaringType == null || (iMethod.DeclaringType.DeclaringType == null ? (string.Format ("{0}.{1}", iface.FullName, iMethod.Name) != tMethod.Name) : (string.Format ("{0}.{1}.{2}", iMethod.DeclaringType.DeclaringType, iface.Name, iMethod.Name) != tMethod.Name))))
 				return false;