aconfig: consolidate how fully qualified flag name is calculated

Add an extension trait to consolidate how the fully qualified package
name (<package>.<name>) is calculated and use this where possible.

This CL is a semantic change only. The intention of this CL is to reduce
the risk of future bugs.

Bug: N/A
Test: atest aconfig.test
Change-Id: Ibab6641dda3843337fbea02631c31a1fdd8fb4ab
diff --git a/tools/aconfig/src/protos.rs b/tools/aconfig/src/protos.rs
index 37eb67d..2684d20 100644
--- a/tools/aconfig/src/protos.rs
+++ b/tools/aconfig/src/protos.rs
@@ -308,7 +308,17 @@
     }
 
     fn create_sorting_key(pf: &ProtoParsedFlag) -> String {
-        format!("{}.{}", pf.package(), pf.name())
+        pf.fully_qualified_name()
+    }
+}
+
+pub trait ParsedFlagExt {
+    fn fully_qualified_name(&self) -> String;
+}
+
+impl ParsedFlagExt for ProtoParsedFlag {
+    fn fully_qualified_name(&self) -> String {
+        format!("{}.{}", self.package(), self.name())
     }
 }