Add the Android interop test App.

So far, it is built separately from the other gRpc components.
diff --git a/.gitignore b/.gitignore
index 89e2e50..e47cc81 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,6 +2,7 @@
 build
 gradle.properties
 .gradle
+local.properties
 
 # IntelliJ IDEA
 .idea