[Xamarin.Android.Build.Tasks] @(LinkDescription) should be in Inputs for the linker (#4816)

Fixes: https://developercommunity.visualstudio.com/content/problem/1061307/changes-not-applied-on-build-after-linker-configur.html

Changes to a `@(LinkDescription)` file did not trigger the
`_LinkAssembliesShrink` MSBuild target to run again.

The only way to see changes from this file was to `Rebuild` every time.

Adding a test for this scenario is also nice, as it didn't seem like
we had an MSBuild test verifying that `@(LinkDescription)` works in
general.

Author: jonathanpeppers

Documentation/release-notes/incremental-linkdescription.md

@@ -0,0 +1,7 @@
+#### Application and library build and deployment
+
+  * [Developer Community 1061307][0]: Fixes an issue where modifying
+    `@(LinkDescription)` files would require a `Rebuild` to see the
+    changes reflected in the Android application build output.
+
+[0]: https://developercommunity.visualstudio.com/content/problem/1061307/changes-not-applied-on-build-after-linker-configur.html

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

@@ -202,5 +202,54 @@ void WarnAboutAppDomains (XamarinAndroidApplicationProject proj, string testName
 				Assert.IsTrue (StringAssertEx.ContainsText (b.LastBuildOutput, "warning XA2000: Use of AppDomain.CreateDomain()"), "Should warn XA2000 about creating AppDomain.");
 			}
 		}
+
+		[Test]
+		public void LinkDescription ()
+		{
+			string linker_xml = "<linker/>";
+
+			var proj = new XamarinAndroidApplicationProject {
+				IsRelease = true,
+				OtherBuildItems = {
+					new BuildItem ("LinkDescription", "linker.xml") {
+						TextContent = () => linker_xml
+					}
+				}
+			};
+			// So we can use Mono.Cecil to open assemblies directly
+			proj.SetProperty ("AndroidEnableAssemblyCompression", "False");
+
+			using (var b = CreateApkBuilder ()) {
+				Assert.IsTrue (b.Build (proj), "first build should have succeeded.");
+
+				linker_xml =
+@"<linker>
+	<assembly fullname=""mscorlib"">
+		<type fullname=""System.Console"">
+			<method name=""Beep"" />
+		</type>
+	</assembly>
+</linker>";
+				proj.Touch ("linker.xml");
+
+				Assert.IsTrue (b.Build (proj, doNotCleanupOnUpdate: true), "second build should have succeeded.");
+
+				var apk = Path.Combine (Root, b.ProjectDirectory, proj.OutputPath, $"{proj.PackageName}.apk");
+				FileAssert.Exists (apk);
+				using (var zip = ZipHelper.OpenZip (apk)) {
+					var entry = zip.ReadEntry ("assemblies/mscorlib.dll");
+					Assert.IsNotNull (entry, "mscorlib.dll should exist in apk!");
+					using (var stream = new MemoryStream ()) {
+						entry.Extract (stream);
+						stream.Position = 0;
+						using (var assembly = AssemblyDefinition.ReadAssembly (stream)) {
+							var type = assembly.MainModule.GetType ("System.Console");
+							var method = type.Methods.FirstOrDefault (p => p.Name == "Beep");
+							Assert.IsNotNull (method, "System.Console.Beep should exist!");
+						}
+					}
+				}
+			}
+		}
 	}
 }

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -1474,7 +1474,7 @@ because xbuild doesn't support framework reference assemblies.
 
 <Target Name="_LinkAssembliesShrink"
   Condition="'$(AndroidLinkMode)' != 'None' and '$(UsingAndroidNETSdk)' != 'true' "
-  Inputs="@(ResolvedUserAssemblies);$(_AndroidBuildPropertiesCache)"
+  Inputs="@(ResolvedUserAssemblies);@(LinkDescription);$(_AndroidBuildPropertiesCache)"
   Outputs="$(_AndroidLinkFlag)">
 
     <PropertyGroup>