chore: ensure thread-safe access to activity timer fields in WebSocketTransport

- marked `lastActivityTime` as volatile, it's low cost, but it will ensure that the timer reads fresh value, otherwise it can read very old value (although it's very unlikely)

- check activity timer before going to the synchronized block

Author: ttypic

lib/src/main/java/io/ably/lib/transport/WebSocketTransport.java

@@ -252,8 +252,8 @@ class WebSocketHandler implements WebSocketListener {
          ***************************/
 
         private final Timer timer = new Timer();
-        private TimerTask activityTimerTask = null;
-        private long lastActivityTime;
+        private volatile TimerTask activityTimerTask = null;
+        private volatile long lastActivityTime;
 
         /**
          * Monitor for activity timer events
@@ -388,6 +388,9 @@ private void checkActivity() {
                 return;
             }
 
+            // prevent going to the synchronized block if the timer is active
+            if (activityTimerTask != null) return;
+
             synchronized (activityTimerMonitor) {
                 // Check if timer already running
                 if (activityTimerTask == null) {