Depend on a module instead of real dir for metalava_merge_annotations_dirs

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: I82e8b51d66f97ebc3f9f4f5aa75ce33066c95089
diff --git a/Android.bp b/Android.bp
index 80afd48..d974b7e 100644
--- a/Android.bp
+++ b/Android.bp
@@ -1142,7 +1142,7 @@
     metalava_annotations_enabled: true,
     metalava_previous_api: ":public-api-for-metalava-annotations",
     metalava_merge_annotations_dirs: [
-        "tools/metalava/manual",
+        "metalava-manual",
     ],
 }