Update databinding exec.jar to 2.3.1.

https://github.com/bazelbuild/bazel/issues/2694

Change-Id: I33e1c1a6c1f1c4b31227eb26b0deed09525a7ec7
diff --git a/third_party/java/android_databinding/BUILD b/third_party/java/android_databinding/BUILD
index 9d66f21..fe821f6 100644
--- a/third_party/java/android_databinding/BUILD
+++ b/third_party/java/android_databinding/BUILD
@@ -28,5 +28,5 @@
 # Executable for the databinding resource compiler.
 java_import(
     name = "exec",
-    jars = ["exec.jar"],
+    jars = ["v2_3_1/exec.jar"],
 )