Add aapt2 to third_party

Content of README.google:

URL: https://dl.google.com/android/maven2/com/android/tools/build/aapt2/8.1.0-alpha01-9289358/aapt2-8.1.0-alpha01-9289358-linux.jar
License: Apache 2
Description: Tool to package android resources
Local Modifications: None.
Change-Id: Ib9929d6d8af4ac636f52167c1b8d1620e674dd01
diff --git a/build.gradle b/build.gradle
index f016148..462e425 100644
--- a/build.gradle
+++ b/build.gradle
@@ -342,6 +342,7 @@
                 "2016-12-19/art"
         ],
         "third_party": [
+                "aapt2",
                 "android_cts_baseline",
                 "android_jar/libcore_latest",
                 "android_jar/lib-v14",