Enable annotations for framework stubs

Keeps Nullable and other source retention annotations in the stubs.

Bug: 157010342
Test: diff_stubs.sh
Change-Id: I4fc66d607276ff885947489afd63e1370ec85939
Merged-In: I4fc66d607276ff885947489afd63e1370ec85939
(cherry picked from commit 01ff6b243febc199ffe718a49374af954c6dbeff)
diff --git a/apex/Android.bp b/apex/Android.bp
index f34ecbd..5ee8a63 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -107,6 +107,8 @@
     // Configure framework module specific metalava options.
     droiddoc_options: [mainline_stubs_args],
 
+    annotations_enabled: true,
+
     // The stub libraries must be visible to frameworks/base so they can be combined
     // into API specific libraries.
     stubs_library_visibility: [