[Xamarin.Android.Build.Tasks] property refactoring/cleanup (dotnet#3623)

Context: dotnet/android@master...jonathanpeppers:fully-incremental

I am working on a revamp of the `<GenerateJavaStubs/>` MSBuild task to
make it "fully incremental".

The following parts of the build would happen on a per-assembly basis:

* Java sources generated
* Java class files compiled via `javac`
* `classes.zip` or `*.jar` files

This will be hidden behind a feature flag such as
`/p:_AndroidFullyIncremental=True` for its first implementation.

The problem I am facing is that our build throws random Java sources
into `$(IntermediateOutputPath)android\src` throughout the build. This
includes files like `R.java`, `MonoRuntimeProvider.java`, etc.

To make it where the location of these files is modifiable, we can
create a few new private properties:

* `$(_AndroidIntermediateJavaSourceDirectory)` which is `$(IntermediateOutputPath)android\src\`
* `$(_AndroidIntermediateJavaClassDirectory)` which is `$(IntermediateOutputPath)android\bin\classes\`
* `$(_AndroidIntermediateClassesZip)` which is `$(IntermediateOutputPath)android\bin\classes.zip`

This way I can modify the location when the `_AndroidFullyIncremental`
feature flag is set.

To refactor things a bit further, I also removed usage of
`$(MonoAndroidIntermediate)` and just used `$(IntermediateOutputPath)`
instead. `$(IntermediateOutputPath)` should be generally more
well-known for MSBuild, and I kept the original property around
because I found usage in other repos.

src/Xamarin.Android.Build.Tasks/MSBuild/Xamarin/Android/Xamarin.Android.Designer.targets

@@ -77,7 +77,7 @@ Copyright (C) 2016 Xamarin. All rights reserved.
 <Target Name="_GeneratePackageManagerJavaForDesigner"
     DependsOnTargets="_AddStaticResources;_ResolveAssemblies;_CopyAssembliesForDesigner;_PrepareAssembliesForDesigner;$(BeforeGenerateAndroidManifest)"
     Inputs="$(_ResolvedUserAssembliesHashFile);@(ResolvedAssemblies);@(ResolvedUserAssemblies);$(_AndroidManifestAbs);"
-    Outputs="$(IntermediateOutputPath)android\src\mono\MonoPackageManager.java;$(_AndroidTypeMappingJavaToManaged);$(_AndroidTypeMappingManagedToJava)">
+    Outputs="$(_AndroidIntermediateJavaSourceDirectory)mono\MonoPackageManager.java;$(_AndroidTypeMappingJavaToManaged);$(_AndroidTypeMappingManagedToJava)">
   <GenerateJavaStubs
       ResolvedAssemblies="@(_ResolvedAssemblies)"
       ResolvedUserAssemblies="@(_ResolvedUserMonoAndroidAssembliesForDesigner)"
@@ -112,8 +112,8 @@ Copyright (C) 2016 Xamarin. All rights reserved.
       ResolvedAssemblies="@(_ResolvedAssemblies)"
       ResolvedUserAssemblies="@(_ResolvedUserAssemblies)"
       MainAssembly="$(MonoAndroidLinkerInputDir)$(TargetFileName)"
-      OutputDirectory="$(IntermediateOutputPath)android\src\mono"
-      EnvironmentOutputDirectory="$(IntermediateOutputPath)android\src\mono\android\app"
+      OutputDirectory="$(_AndroidIntermediateJavaSourceDirectory)mono"
+      EnvironmentOutputDirectory="$(_AndroidIntermediateJavaSourceDirectory)mono\android\app"
       UseSharedRuntime="$(AndroidUseSharedRuntime)"
       TargetFrameworkVersion="$(TargetFrameworkVersion)"
       Manifest="$(IntermediateOutputPath)android\AndroidManifest.xml"
@@ -139,9 +139,9 @@ Copyright (C) 2016 Xamarin. All rights reserved.
     Outputs="$(_AndroidStampDirectory)SetupDependenciesForDesigner.stamp">
   <Javac
       JavaPlatformJarPath="$(JavaPlatformJarPath)"
-      ClassesOutputDirectory="$(IntermediateOutputPath)android\bin\classes"
+      ClassesOutputDirectory="$(_AndroidIntermediateJavaClassDirectory)"
       TargetFrameworkDirectory="$(TargetFrameworkDirectory)"
-      StubSourceDirectory="$(IntermediateOutputPath)android\src"
+      StubSourceDirectory="$(_AndroidIntermediateJavaSourceDirectory)"
       JavaSourceFiles=""
       ToolPath="$(JavacToolPath)"
       ToolExe="$(JavacToolExe)"