Come up with an alternate way for java-interop.jar path detection

Context: https://xamarinhq.slack.com/archives/C03CEGRUW/p1568076852178100

Using `$(MSBuildExtensionsPath)` didn't work, as it just took the
"first" path, which isn't valid, resulting in attempts to use:

	/Library/Frameworks/Mono.framework/Versions/6.0.0/lib/mono/xbuild/Xamarin/Android/java-interop.jar

which does not exist.

build-tools/scripts/Jar.targets

@@ -13,6 +13,7 @@
   </Target>
   <Target Name="BuildTestJarFile"
       DependsOnTargets="_GetJavacVersions"
+      DependsOnTargets="_GetJavacVersions;_CollectRuntimeJarFilenames"
       Inputs="@(TestJarEntry)"
       Outputs="%(TestJarEntry.OutputFile)">
     <ItemGroup>
@@ -27,7 +28,7 @@
       <_DestDir>$(IntermediateOutputPath)__CreateTestJarFile-bin</_DestDir>
       <_AndroidJar>-bootclasspath "$(AndroidSdkDirectory)\platforms\android-$(_AndroidApiLevelName)\android.jar"</_AndroidJar>
       <_JIJar_InTree>$([System.IO.Path]::GetFullPath ('$(XAInstallPrefix)'))\xbuild\Xamarin\Android\java-interop.jar</_JIJar_InTree>
-      <_JIJar_System>$(MSBuildExtensionsPath)\Xamarin\Android\java-interop.jar</_JIJar_System>
+      <_JIJar_System>$([System.IO.Path]::GetDirectoryName ($(_RuntimeDex)))\java-interop.jar</_JIJar_System>
       <_JIJar Condition=" Exists ($(_JIJar_InTree)) ">$(_JIJar_InTree)</_JIJar>
       <_JIJar Condition=" '$(_JIJar)' == '' ">$(_JIJar_System)</_JIJar>
       <_CP>-cp "$(_JIJar)"</_CP>