[Xamarin.Android.Build.Tasks] Fix an issue where library projects cannot be debugged.

The linker was not producing ppdb files.
What is supposed to happen is the
linker if it changes a dll should emit a new pdb file
along with it. Because none of the assemblies were being
loaded with debug symbols. Hence no updated pdb.

So this commit enables the reading of Symbols as part of the
linker process to ensure that we are able to emit the new
pdb when required.

Author: dellis1972

src/Xamarin.Android.Build.Tasks/Tasks/LinkAssemblies.cs

@@ -84,8 +84,9 @@ public override bool Execute ()
 
 			var rp = new ReaderParameters {
 				InMemory    = true,
+				ReadSymbols = true,
 			};
-			using (var res = new DirectoryAssemblyResolver (Log.LogWarning, loadDebugSymbols: false, loadReaderParameters: rp)) {
+			using (var res = new DirectoryAssemblyResolver (Log.LogWarning, loadDebugSymbols: true, loadReaderParameters: rp)) {
 				return Execute (res);
 			}
 		}

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

@@ -499,6 +499,70 @@ public void BuildBasicApplicationCheckMdbRepeatBuild ()
 			}
 		}
 
+		[Test]
+		public void BuildAppCheckDebugSymbols ()
+		{
+			var path = Path.Combine ("temp", TestContext.CurrentContext.Test.Name);
+			var lib = new XamarinAndroidLibraryProject () {
+				IsRelease = false,
+				ProjectName = "Library1",
+				Sources = {
+					new BuildItem.Source ("Class1.cs") {
+						TextContent = () => @"using System;
+namespace Library1 {
+	public class Class1 : Java.Lang.Object, global::Android.Views.View.IOnClickListener {
+		void global::Android.Views.View.IOnClickListener.OnClick(global::Android.Views.View v)
+		{
+		}
+	}
+}
+",
+					},
+				},
+			};
+			var proj = new XamarinAndroidApplicationProject () {
+				IsRelease = false,
+				ProjectName = "App1",
+				References = { new BuildItem ("ProjectReference", "..\\Library1\\Library1.csproj") },
+				Sources = {
+					new BuildItem.Source ("Class2.cs") {
+						TextContent= () => @"
+using System;
+namespace App1
+{
+	public class Class2
+	{
+		Library1.Class1 c;
+		public Class2 ()
+		{
+		}
+	}
+}"
+					},
+				},
+			};
+			proj.SetProperty (KnownProperties.AndroidLinkMode, AndroidLinkMode.None.ToString ());
+			using (var libb = CreateDllBuilder (Path.Combine (path, "Library1"))) {
+				Assert.IsTrue (libb.Build (lib), "Library1 Build should have succeeded.");
+				using (var b = CreateApkBuilder (Path.Combine (path, "App1"))) {
+					Assert.IsTrue (b.Build (proj), "App1 Build should have succeeded.");
+					var assetsPdb = Path.Combine (Root, b.ProjectDirectory, proj.IntermediateOutputPath, "android", "assets", "Library1.pdb");
+					var linkDst = Path.Combine (Root, b.ProjectDirectory, proj.IntermediateOutputPath, "linkdst", "Library1.pdb");
+					var linkSrc = Path.Combine (Root, b.ProjectDirectory, proj.IntermediateOutputPath, "linksrc", "Library1.pdb");
+					Assert.IsTrue (
+						File.Exists (assetsPdb),
+						"Library1.pdb must be copied to Intermediate directory");
+					Assert.IsTrue (
+						File.Exists (linkDst),
+						"Library1.pdb must be copied to linkdst directory");
+					Assert.IsTrue (
+						File.Exists (linkSrc),
+						"Library1.pdb must be copied to linksrc directory");
+					FileAssert.AreEqual (linkDst, assetsPdb, $"Library1.pdb in {assetsPdb} should match {linkDst}");
+				}
+			}
+		}
+
 		[Test]
 		public void BuildBasicApplicationCheckMdbAndPortablePdb ()
 		{