Add gradle license headers

Summary: Some more will follow.

Reviewed By: jknoxville

Differential Revision: D15146750

fbshipit-source-id: b78331b1fb00f357812e598165d6def62e2a89c8

android/build.gradle

@@ -1,3 +1,8 @@
+// Copyright (c) Facebook, Inc. and its affiliates.
+//
+// This source code is licensed under the MIT license found in the LICENSE file
+// in the root directory of this source tree.
+
 apply plugin: 'com.android.library'
 apply plugin: 'maven'
 

android/sample/build.gradle

@@ -1,3 +1,8 @@
+// Copyright (c) Facebook, Inc. and its affiliates.
+//
+// This source code is licensed under the MIT license found in the LICENSE file
+// in the root directory of this source tree.
+
 apply plugin: 'com.android.application'
 
 android {

build.gradle

@@ -1,3 +1,8 @@
+// Copyright (c) Facebook, Inc. and its affiliates.
+//
+// This source code is licensed under the MIT license found in the LICENSE file
+// in the root directory of this source tree.
+
 buildscript {
     repositories {
         google()