asset: add missing license header

Change-Id: Ife81591606a2fadcc216eae0ea17d772b3757cf7
Reviewed-on: https://go-review.googlesource.com/11736
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

asset/asset_android.go

@@ -1,3 +1,7 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 package asset
 
 /*