Add android.jar for Anderoid API level 21

Content of README.google:
-------------------------

Name: Android system library jar
URL: https://developer.android.com/studio/index.html
Version: API version 21
Revision:NA
Date: June 23th 2017
License: Apache License Version 2.0

Description:
This is the android.jar pulled using the android sdk manager.
Change-Id: I64214b1e4cc1b3f50924547ce6cbc76745f96b8c
diff --git a/build.gradle b/build.gradle
index dfb4623..d8402bc 100644
--- a/build.gradle
+++ b/build.gradle
@@ -109,6 +109,7 @@
         "third_party": [
                 "android_jar/lib-v14.tar.gz",
                 "android_jar/lib-v19.tar.gz",
+                "android_jar/lib-v21.tar.gz",
                 "android_jar/lib-v24.tar.gz",
                 "android_jar/lib-v25.tar.gz",
                 "android_jar/lib-v26.tar.gz",