[Xamarin.Android.Build.Tasks] move <UsingTask/> to ref'ing .targets (#…

…4776)

We have split up `$(AndroidDexTool)` implementations into:

  * `Xamarin.Android.D8.targets`
  * `Xamarin.Android.DX.targets`

Move the `<UsingTask/>` declarations to these files as well.

There was also a reference to a `Xamarin.Android.Tasks.Dx` task that
doesn't exist; I removed it.

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

@@ -42,7 +42,6 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
 <UsingTask TaskName="Xamarin.Android.Tasks.CalculateProjectDependencies" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.CheckForRemovedItems" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.CheckForInvalidResourceFileNames" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
-<UsingTask TaskName="Xamarin.Android.Tasks.CompileToDalvik" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.CollectPdbFiles" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.CollectNonEmptyDirectories" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.ComputeHash" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
@@ -73,9 +72,6 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
 <UsingTask TaskName="Xamarin.Android.Tasks.GetImportedLibraries" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.GetMonoPlatformJar" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.Javac" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
-<UsingTask TaskName="Xamarin.Android.Tasks.Dx" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
-<UsingTask TaskName="Xamarin.Android.Tasks.D8" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
-<UsingTask TaskName="Xamarin.Android.Tasks.R8" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.KeyTool" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.LinkAssemblies" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.LinkAssembliesNoShrink" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
@@ -102,9 +98,7 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
 <UsingTask TaskName="Xamarin.Android.Tasks.CollectLibraryAssets" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.ParseAndroidWearProjectAndManifest" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.PrepareWearApplicationFiles" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
-<UsingTask TaskName="Xamarin.Android.Tasks.Proguard" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.DetermineJavaLibrariesToCompile" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
-<UsingTask TaskName="Xamarin.Android.Tasks.CreateMultiDexMainDexClassList" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.CompileNativeAssembly" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.LinkApplicationSharedLibraries" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.PrepareAbiItems" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />

src/Xamarin.Android.Build.Tasks/Xamarin.Android.D8.targets

@@ -14,6 +14,9 @@ Copyright (C) 2018 Xamarin. All rights reserved.
 
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
 
+  <UsingTask TaskName="Xamarin.Android.Tasks.D8" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
+  <UsingTask TaskName="Xamarin.Android.Tasks.R8" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
+
   <Target Name="_CompileToDalvik"
       DependsOnTargets="$(_BeforeCompileToDalvik);$(_CompileToDalvikDependsOnTargets)"
       Inputs="$(_CompileToDalvikInputs)"

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

@@ -14,6 +14,10 @@ Copyright (C) 2018 Xamarin. All rights reserved.
 
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
 
+  <UsingTask TaskName="Xamarin.Android.Tasks.CompileToDalvik"                AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
+  <UsingTask TaskName="Xamarin.Android.Tasks.CreateMultiDexMainDexClassList" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
+  <UsingTask TaskName="Xamarin.Android.Tasks.Proguard"                       AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
+
   <Target Name="_CompileToDalvik"
       DependsOnTargets="$(_BeforeCompileToDalvik);$(_CompileToDalvikDependsOnTargets)"
       Inputs="$(_CompileToDalvikInputs)"