Don't show staged value if flag is RO

Bug: 324436145
Change-Id: I493421f29d6e7f5e844dd47cc6417b3d5fe9cdbb
Test: m aflags && aflags list
diff --git a/tools/aconfig/aflags/src/main.rs b/tools/aconfig/aflags/src/main.rs
index a726cc0..07b7243 100644
--- a/tools/aconfig/aflags/src/main.rs
+++ b/tools/aconfig/aflags/src/main.rs
@@ -116,9 +116,10 @@
     }
 
     fn display_staged_value(&self) -> String {
-        match self.staged_value {
-            Some(v) => format!("(->{})", v),
-            None => "-".to_string(),
+        match (&self.permission, self.staged_value) {
+            (FlagPermission::ReadOnly, _) => "-".to_string(),
+            (FlagPermission::ReadWrite, None) => "-".to_string(),
+            (FlagPermission::ReadWrite, Some(v)) => format!("(->{})", v),
         }
     }
 }