Turn droiddoc's metalava_merge_annotations_dir into a list.

This change replaces droiddoc's metalava_merge_annotations_dir
parameter (which takes a single value) with
metalava_merge_annotations_dirs (which takes a list). This will makes
it possible to merge libcore annotations in from a separate directory
under ojluni, instead of adding them in a preprocessing step.

Test: `make metalava-api-stubs-docs` with a local change to add a second directory
Bug: 111639530
Change-Id: I8eb778038c1cb52ddb6fc86d972f555f49e836b2
diff --git a/Android.bp b/Android.bp
index c682822..80afd48 100644
--- a/Android.bp
+++ b/Android.bp
@@ -1141,7 +1141,9 @@
     metalava_enabled: true,
     metalava_annotations_enabled: true,
     metalava_previous_api: ":public-api-for-metalava-annotations",
-    metalava_merge_annotations_dir: "tools/metalava/manual",
+    metalava_merge_annotations_dirs: [
+        "tools/metalava/manual",
+    ],
 }
 
 droiddoc {