Use getters for DatastoreOptions and the Datastore service (#401)

`defaultInstance()` and `service()` seem to be deprecated in favor of `getDefaultInstance()` and `getService()`

Author: glaforge

samples/snippets/src/main/java/com/example/datastore/QuickstartSample.java

@@ -26,7 +26,7 @@
 public class QuickstartSample {
   public static void main(String... args) throws Exception {
     // Instantiates a client
-    Datastore datastore = DatastoreOptions.defaultInstance().service();
+    Datastore datastore = DatastoreOptions.getDefaultInstance().getService();
 
     // The kind for the new entity
     String kind = "Task";