Only add vendor_a to microdroid_super if vendor modules flag is disabled

Bug: 307904471
Test: inspect microdroid_super.img on aosp_cheetah-trunk-userdebug build
Test: verify it contains vendor_a partition
Change-Id: If8962ed8e0e9f88af7460f9d7f5ce2f26aa90c68
diff --git a/microdroid/Android.bp b/microdroid/Android.bp
index a1ce594..f134eef 100644
--- a/microdroid/Android.bp
+++ b/microdroid/Android.bp
@@ -283,12 +283,14 @@
     ],
     soong_config_variables: {
         release_avf_enable_vendor_modules: {
-            default_group: [
-                {
-                    name: "vendor_a",
-                    filesystem: ":microdroid_vendor",
-                },
-            ],
+            conditions_default: {
+                default_group: [
+                    {
+                        name: "vendor_a",
+                        filesystem: ":microdroid_vendor",
+                    },
+                ],
+            },
         },
     },
 }