storaged: fix regression found in unit-test

Should use reference instead of copy to clear storaged proto

Bug: 63740245
Change-Id: Ibb933a0bac12508467fc104b37e7cc490dfbe948
diff --git a/storaged/storaged_uid_monitor.cpp b/storaged/storaged_uid_monitor.cpp
index e0a5d01..ab1c16e 100644
--- a/storaged/storaged_uid_monitor.cpp
+++ b/storaged/storaged_uid_monitor.cpp
@@ -407,7 +407,7 @@
 
 void uid_monitor::update_uid_io_proto(unordered_map<int, StoragedProto>* protos)
 {
-    for (auto it : *protos) {
+    for (auto& it : *protos) {
         it.second.mutable_uid_io_usage()->Clear();
     }