libvbmeta: add getter for rollback_index

This will be used by the derive_microdroid_vendor_dice_node binary in
the follow-up patch.

Bug: 287593065
Test: atest libvbmeta_rust.test
Change-Id: Ia38579a1038514190b98a5f1c82bab3bb509078b
diff --git a/tests/vendor_images/Android.bp b/tests/vendor_images/Android.bp
index 26dbc01..66f0219 100644
--- a/tests/vendor_images/Android.bp
+++ b/tests/vendor_images/Android.bp
@@ -15,6 +15,16 @@
     file_contexts: ":microdroid_vendor_file_contexts.gen",
     use_avb: true,
     avb_private_key: ":vendor_sign_key",
+    rollback_index: 5,
+}
+
+android_filesystem {
+    name: "test_microdroid_vendor_image_no_rollback_index",
+    partition_name: "microdroid-vendor",
+    type: "ext4",
+    file_contexts: ":microdroid_vendor_file_contexts.gen",
+    use_avb: true,
+    avb_private_key: ":vendor_sign_key",
 }
 
 android_filesystem {