Move .annotated.java files into an 'sdk' subdirectory.

All of the current annotations are intended for use in the SDK build,
and live at ojluni/annotations. In order to make room for annotated
stubs for other purposes, this change moves them. There is a README
that describes the intended directory structure. (N.B. Validation of
the nullability stubs is not landed yet but is described here in
anticipation.)

Bug: 115746226
Test: `make api-stubs-docs`
Change-Id: I7cfbdb5dc9888f796eb1f384155fce33125a7d9e
diff --git a/JavaLibrary.bp b/JavaLibrary.bp
index 21645d3..1df3ea3 100644
--- a/JavaLibrary.bp
+++ b/JavaLibrary.bp
@@ -484,8 +484,8 @@
 
 // Make the annotated stubs in ojluni/annotations available to metalava:
 droiddoc_exported_dir {
-    name: "ojluni-annotated-stubs",
-    path: "ojluni/annotations",
+    name: "ojluni-annotated-sdk-stubs",
+    path: "ojluni/annotations/sdk",
 }
 
 // A file containing the list of tags that are "known" to us from the OpenJdk