Use 28.txt as API file for migrating NULLs in Metalava

Use last released API file for annotations reference in master branch
(non-releasing branch).

Test: manual
Bug: b/78245848
Change-Id: I0520231619001032c00d98fdc8cd86aa41f4fa14

Android.bp

@@ -1571,7 +1571,7 @@ doc_defaults {
     installable: false,
     metalava_enabled: true,
     metalava_annotations_enabled: true,
-    metalava_previous_api: ":public-api-for-metalava-annotations",
+    metalava_previous_api: ":last-released-public-api",
     metalava_merge_annotations_dirs: [
         "metalava-manual",
         "ojluni-annotated-stubs",