Expose Bazel's Android data binding support to users.

The logic is already in Bazel but wasn't available to build rules.
This change makes it available, but still requires data binding's
{build|run}time libraries to be checked into appropriate depot
spots for everything to work.

Followup changes will make those libraries easily available.

Issue: #2694
PiperOrigin-RevId: 153359861
diff --git a/tools/android/BUILD.tools b/tools/android/BUILD.tools
index 0d684d0..782fea9 100644
--- a/tools/android/BUILD.tools
+++ b/tools/android/BUILD.tools
@@ -205,3 +205,8 @@
     name = "debug_keystore",
     srcs = ["bazel_debug.keystore"],
 )
+
+alias(
+    name = "databinding_annotation_processor",
+    actual = "@android_databinding//:annotation_processor",
+)