[tests] fix for ANDROID_SDK_PATH (#211)

Context: https://github.com/xamarin/xamarin-android/blob/f56ca4ab05471bcea8225f348c6cbd793ccbe9cb/build-tools/scripts/RunTests.targets#L50
Context: https://devdiv.visualstudio.com/DevDiv/Default/_build/index?buildId=1138588&_a=summary&tab=ms.vss-test-web.test-result-details

On Windows, the Java.Interop tests are not getting the `$ANDROID_SDK_PATH`
environment variable set. This is somehow working on macOS, but I think
it might be simpler in this case to pass in the `$(AndroidSdkDirectory)`
property from `RunTests.targets` in xamarin-android.

This approach worked when testing locally. I will need to bump Java.Interop
and make one further change in xamarin-android to pass in
`$(AndroidSdkDirectory)`. This will get a few skipped tests in Java.Interop
running and passing on Windows.

Author: jonathanpeppers

build-tools/scripts/RunNUnitTests.targets

@@ -23,7 +23,8 @@
         DestinationFolder="$(_TopDir)\bin\Test$(Configuration)"
         SkipUnchangedFiles="True"
     />
-    <SetEnvironmentVariable Name="MONO_TRACE_LISTENER" Value="Console.Out" />
+    <SetEnvironmentVariable Name="ANDROID_SDK_PATH"      Value="$(AndroidSdkDirectory)" Condition=" '$(AndroidSdkDirectory)' != '' " />
+    <SetEnvironmentVariable Name="MONO_TRACE_LISTENER"   Value="Console.Out" />
     <SetEnvironmentVariable Name="JAVA_INTEROP_GREF_LOG" Value="g-%(_TestAssembly.Filename).txt" />
     <SetEnvironmentVariable Name="JAVA_INTEROP_LREF_LOG" Value="l-%(_TestAssembly.Filename).txt" />
     <Exec