Update google-cloud-spanner/src/main/java/com/google/cloud/spanner/Tr…

…ansactionRunnerImpl.java

Co-authored-by: Knut Olav Løite <[email protected]>

google-cloud-spanner/src/main/java/com/google/cloud/spanner/TransactionRunnerImpl.java

@@ -197,7 +197,7 @@ public void removeListener(Runnable listener) {
     volatile ByteString transactionId;
 
     private CommitResponse commitResponse;
-    private Clock clock = new Clock();
+    private final Clock clock;
 
     private TransactionContextImpl(Builder builder) {
       super(builder);