[Xamarin.Android.Build.Tasks] Fix Removal of Non Duplicate elements

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=59473

Commit d079a42 went a bit too far when removing duplicates.
It removed ANY duplicate entry that appeared anywhere in the
document. What we really wanted to remove full duplicates that
exist at the same level as the current element in the document.
For example

```xml
	<foo>
		<bar name="bar1">
		  <bar2 />
		</bar>
                <bar name="bar2">
                  <bar2 />
                </bar>
		<dupe/>
		<dupe/>
	</foo>
```

`bar2` should NOT be removed but one of the `dupe` values should.
So this commit reworks the RemoveDuplicates code to handle the
correct logic. It also adds a unit test for this exact senario.

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/ManifestTest.cs

@@ -544,7 +544,56 @@ public void MergeLibraryManifest ()
 				References = {
 					new BuildItem.ProjectReference ("..\\Binding1\\Binding1.csproj", lib.ProjectGuid)
 				},
+				Packages = {
+					KnownPackages.SupportMediaCompat_25_4_0_1,
+					KnownPackages.SupportFragment_25_4_0_1,
+					KnownPackages.SupportCoreUtils_25_4_0_1,
+					KnownPackages.SupportCoreUI_25_4_0_1,
+					KnownPackages.SupportCompat_25_4_0_1,
+					KnownPackages.AndroidSupportV4_25_4_0_1,
+					KnownPackages.SupportV7AppCompat_25_4_0_1,
+				},
 			};
+			proj.Sources.Add (new BuildItem.Source ("TestActivity1.cs") {
+				TextContent = () => @"using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+using Android.App;
+using Android.Content;
+using Android.OS;
+using Android.Runtime;
+using Android.Views;
+using Android.Widget;
+using Android.Support.V4.App;
+using Android.Util;
+[Activity (Label = ""TestActivity1"")]
+[IntentFilter (new[]{Intent.ActionMain}, Categories = new[]{ ""com.xamarin.sample"" })]
+public class TestActivity1 : FragmentActivity {
+}
+				",
+			});
+			proj.Sources.Add (new BuildItem.Source ("TestActivity2.cs") {
+				TextContent = () => @"using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+using Android.App;
+using Android.Content;
+using Android.OS;
+using Android.Runtime;
+using Android.Views;
+using Android.Widget;
+using Android.Support.V4.App;
+using Android.Util;
+[Activity (Label = ""TestActivity2"")]
+[IntentFilter (new[]{Intent.ActionMain}, Categories = new[]{ ""com.xamarin.sample"" })]
+public class TestActivity2 : FragmentActivity {
+}
+				",
+			});
 			using (var libbuilder = CreateDllBuilder (Path.Combine (path, "Binding1"))) {
 				Assert.IsTrue (libbuilder.Build (lib), "Build should have succeeded.");
 				using (var builder = CreateApkBuilder (Path.Combine (path, "App1"))) {
@@ -558,6 +607,15 @@ public void MergeLibraryManifest ()
 						".internal.FacebookInitProvider was not replaced with com.xamarin.test.internal.FacebookInitProvider");
 					Assert.AreEqual (manifest.IndexOf ("meta-data", StringComparison.OrdinalIgnoreCase),
 					                 manifest.LastIndexOf ("meta-data", StringComparison.OrdinalIgnoreCase), "There should be only one meta-data element");
+
+					var doc = XDocument.Parse (manifest);
+					var ns = XNamespace.Get ("http://schemas.android.com/apk/res/android");
+
+					var activities = doc.Element ("manifest")?.Element ("application")?.Elements ("activity");
+					var e = activities.FirstOrDefault (x => x.Attribute (ns.GetName ("label"))?.Value == "TestActivity2");
+					Assert.IsNotNull (e, "Manifest should contain an activity for TestActivity2");
+					Assert.IsNotNull (e.Element ("intent-filter"), "TestActivity2 should have an intent-filter");
+					Assert.IsNotNull (e.Element ("intent-filter").Element ("action"), "TestActivity2 should have an intent-filter/action");
 				}
 			}
 		}

src/Xamarin.Android.Build.Tasks/Utilities/ManifestDocument.cs

@@ -402,11 +402,18 @@ void MergeLibraryManifest (string mergedManifest)
 			}
 		}
 
+		public IEnumerable<XElement> ResolveDuplicates (IEnumerable<XElement> elements)
+		{
+			foreach (var e in elements)
+				foreach (var d in ResolveDuplicates (e.Elements ()))
+					yield return d;
+			foreach (var d in elements.GroupBy (x => x.ToFullString ()).SelectMany (x => x.Skip (1)))
+				yield return d;
+		}
+
 		void RemoveDuplicateElements ()
 		{
-			var duplicates = doc.Descendants ()
-			                    .GroupBy (x => x.ToFullString ())
-					    .SelectMany (x => x.Skip (1));
+			var duplicates = ResolveDuplicates (doc.Elements ());
 			foreach (var duplicate in duplicates)
 				duplicate.Remove ();