Added new Android build target and makefile utils.

Previously Android.mk only had a rule for the Flatbuffers test. There
is now an empty module definition so that flatbuffers can be included
as an Android module in other projects.

Additionally, android/jni/include.mk has been added which contains
some utility functions that can be used by projects using Flatbuffers
to generate header build rules and set up dependencies.

A sample project has been added to the samples directory to
demonstrate how to use flatbuffers with Android.

Tested by compiling Android project on Linux.

Change-Id: I25d6da40f6531777b22f7371187e0a2f4e903ad4

samples/android/jni/main.cpp

@@ -0,0 +1,28 @@
+// Copyright 2015 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "android_native_app_glue.h"
+#include "animal_generated.h"
+#include "flatbuffers/flatbuffers.h"
+
+void android_main(android_app *app) {
+  app_dummy();
+
+  flatbuffers::FlatBufferBuilder builder;
+  auto name = builder.CreateString("Dog");
+  auto sound = builder.CreateString("Bark");
+  auto animal_buffer = sample::CreateAnimal(builder, name, sound);
+  builder.Finish(animal_buffer);
+}
+