Possible NPE in PayloadController.shutdown

[hieuletrung]

In https://github.com/newrelic/newrelic-android-agent/blob/main/agent-core/src/main/java/com/newrelic/agent/android/payload/PayloadController.java#L112, there's possibility of NPE araise. Since the queueExecutor init here https://github.com/newrelic/newrelic-android-agent/blob/main/agent-core/src/main/java/com/newrelic/agent/android/payload/PayloadController.java#L76, but it doesn't gurantee to be atomic.
After we have the getAndSet new instance here https://github.com/newrelic/newrelic-android-agent/blob/main/agent-core/src/main/java/com/newrelic/agent/android/payload/PayloadController.java#L72, the isInitialized() will return true since the instance is available after that call

Here is stack trace of the NPE.

java.lang.NullPointerException: Attempt to invoke virtual method 'void com.newrelic.agent.android.payload.PayloadController.shutdown()' on a null object reference 
at com.newrelic.agent.android.payload.PayloadController.shutdown(PayloadController.java:112) 
at com.newrelic.agent.android.AndroidAgentImpl.stop(AndroidAgentImpl.java:550) 
at com.newrelic.agent.android.AndroidAgentImpl.stop(AndroidAgentImpl.java:473) 
at com.newrelic.agent.android.AndroidAgentImpl.applicationBackgrounded(AndroidAgentImpl.java:629) 
at com.newrelic.agent.android.background.ApplicationStateMonitor.notifyApplicationInBackground(ApplicationStateMonitor.java:108) 
at com.newrelic.agent.android.background.ApplicationStateMonitor.-911900Nest(Unknown Source:0) 
at com.newrelic.agent.android.background.ApplicationStateMonitor.run(ApplicationStateMonitor.java:65) 
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167) 
at java.util.concurrent.ThreadPoolExecutor.run(ThreadPoolExecutor.java:641) 
at java.lang.Thread.run(Thread.java:923)