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/third_party/aapt2.tar.gz.sha1 b/third_party/aapt2.tar.gz.sha1
new file mode 100644
index 0000000..93bac6b
--- /dev/null
+++ b/third_party/aapt2.tar.gz.sha1
@@ -0,0 +1 @@
+b073e3c0ad3b0bc74425e61b372c163f664bc3fd
\ No newline at end of file