Fix return-stack-address warning

hardware/interfaces/security/keymint/aidl/vts/performance/KeyMintBenchmark.cpp:79:26: error: reference to stack memory associated with local variable 'message' returned [-Werror,-Wreturn-stack-address]
return std::move(message);
^~~~~~~

Test: presubmit
Change-Id: I4298b4a25ccb809a7ae180bb218e673a7f1aa623
diff --git a/security/keymint/aidl/vts/performance/KeyMintBenchmark.cpp b/security/keymint/aidl/vts/performance/KeyMintBenchmark.cpp
index 0c61c25..49fd0c9 100644
--- a/security/keymint/aidl/vts/performance/KeyMintBenchmark.cpp
+++ b/security/keymint/aidl/vts/performance/KeyMintBenchmark.cpp
@@ -68,7 +68,7 @@
 
     int getError() { return static_cast<int>(error_); }
 
-    const string& GenerateMessage(int size) {
+    const string GenerateMessage(int size) {
         for (const string& message : message_cache_) {
             if (message.size() == size) {
                 return message;
@@ -76,7 +76,7 @@
         }
         string message = string(size, 'x');
         message_cache_.push_back(message);
-        return std::move(message);
+        return message;
     }
 
     optional<BlockMode> getBlockMode(string transform) {