Add new module to track deps in metalava/manual

Bug: b/111916275
Test: touch manual/android/support/design/widget/annotations.xml, and m
-j metalava-api-stubs-docs, and check the built output srcjar.

Change-Id: I683fc7559d896742da958bb5a9d5ee92ad593459
diff --git a/Android.bp b/Android.bp
index edbfb08..644cbf7 100644
--- a/Android.bp
+++ b/Android.bp
@@ -60,3 +60,8 @@
     ],
     no_framework_libs: true,
 }
+
+droiddoc_exported_dir {
+    name: "metalava-manual",
+    path: "manual",
+}