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/Application.mk

@@ -0,0 +1,22 @@
+# Copyright (c) 2014 Google, Inc.
+#
+# This software is provided 'as-is', without any express or implied
+# warranty.  In no event will the authors be held liable for any damages
+# arising from the use of this software.
+# Permission is granted to anyone to use this software for any purpose,
+# including commercial applications, and to alter it and redistribute it
+# freely, subject to the following restrictions:
+# 1. The origin of this software must not be misrepresented; you must not
+# claim that you wrote the original software. If you use this software
+# in a product, an acknowledgment in the product documentation would be
+# appreciated but is not required.
+# 2. Altered source versions must be plainly marked as such, and must not be
+# misrepresented as being the original software.
+# 3. This notice may not be removed or altered from any source distribution.
+APP_PLATFORM := android-10
+APP_PROJECT_PATH := $(call my-dir)/..
+APP_STL := gnustl_static
+
+APP_ABI := armeabi-v7a
+NDK_TOOLCHAIN_VERSION := 4.8
+APP_CPPFLAGS += -std=c++11