Add an example for onPayloadReady and VM service

This adds a simple integer addition service to the testapk's binary. The
demo app tries connecting the VM service once onPayloadReady is
triggered. Users can see the VM service's output.

Bug: 195381416
Test: launch MicrodroidDemoApp
Test: atest MicrodroidHostTestCases
Change-Id: I346084d08f753772cc00aa4c052e0b9b41d460ce
diff --git a/demo/Android.bp b/demo/Android.bp
index 37fc676..749ca90 100644
--- a/demo/Android.bp
+++ b/demo/Android.bp
@@ -9,6 +9,7 @@
     static_libs: [
         "androidx-constraintlayout_constraintlayout",
         "androidx.appcompat_appcompat",
+        "com.android.microdroid.testservice-java",
         "com.google.android.material_material",
     ],
     libs: [