xds: Fix flow control data race in ControlPlaneClient

As discovered by TSAN, the adsStream field is not synchronized.
```
WARNING: ThreadSanitizer: data race (pid=1625)
  Read of size 4 at 0x00009b66fc88 by thread T23 (mutexes: write M0):
    #0 io.grpc.xds.ControlPlaneClient.isReady()Z ControlPlaneClient.java:203
    #1 io.grpc.xds.ControlPlaneClient.readyHandler()V ControlPlaneClient.java:211
    #2 io.grpc.xds.ControlPlaneClient$AdsStream.onReady()V ControlPlaneClient.java:328
    #3 io.grpc.xds.GrpcXdsTransportFactory$EventHandlerToCallListenerAdapter.onReady()V GrpcXdsTransportFactory.java:145
    #4 io.grpc.PartialForwardingClientCallListener.onReady()V PartialForwardingClientCallListener.java:44
    #5 io.grpc.ForwardingClientCallListener.onReady()V ForwardingClientCallListener.java:23
    #6 io.grpc.ForwardingClientCallListener$SimpleForwardingClientCallListener.onReady()V ForwardingClientCallListener.java:40
    #7 io.grpc.PartialForwardingClientCallListener.onReady()V PartialForwardingClientCallListener.java:44
    #8 io.grpc.ForwardingClientCallListener.onReady()V ForwardingClientCallListener.java:23
    #9 io.grpc.ForwardingClientCallListener$SimpleForwardingClientCallListener.onReady()V ForwardingClientCallListener.java:40
    #10 io.grpc.internal.DelayedClientCall$DelayedListener.onReady()V DelayedClientCall.java:497
    #11 io.grpc.internal.ClientCallImpl$ClientStreamListenerImpl$1StreamOnReady.runInternal()V ClientCallImpl.java:781
    #12 io.grpc.internal.ClientCallImpl$ClientStreamListenerImpl$1StreamOnReady.runInContext()V ClientCallImpl.java:772
    #13 io.grpc.internal.ContextRunnable.run()V ContextRunnable.java:37
    #14 io.grpc.internal.SerializingExecutor.run()V SerializingExecutor.java:133
    #15 java.util.concurrent.ThreadPoolExecutor.runWorker(Ljava/util/concurrent/ThreadPoolExecutor$Worker;)V ThreadPoolExecutor.java:1130
    #16 java.util.concurrent.ThreadPoolExecutor$Worker.run()V ThreadPoolExecutor.java:630
    #17 java.lang.Thread.run()V Thread.java:830
    #18 (Generated Stub) <null>

  Previous write of size 4 at 0x00009b66fc88 by thread T4 (mutexes: write M1, write M2, write M3, write M4, write M5):
    #0 io.grpc.xds.ControlPlaneClient$AdsStream.cleanUp()V ControlPlaneClient.java:424
    #1 io.grpc.xds.ControlPlaneClient$AdsStream.close(Ljava/lang/Exception;)V ControlPlaneClient.java:418
    #2 io.grpc.xds.ControlPlaneClient$1.run()V ControlPlaneClient.java:130
    #3 io.grpc.SynchronizationContext.drain()V SynchronizationContext.java:94
    #4 io.grpc.SynchronizationContext.execute(Ljava/lang/Runnable;)V SynchronizationContext.java:126
    #5 io.grpc.xds.XdsClientImpl.shutdown()V XdsClientImpl.java:207
    #6 io.grpc.xds.SharedXdsClientPoolProvider$RefCountedXdsClientObjectPool.returnObject(Ljava/lang/Object;)Lio/grpc/xds/XdsClient; SharedXdsClientPoolProvider.java:144
    #7 io.grpc.xds.SharedXdsClientPoolProvider$RefCountedXdsClientObjectPool.returnObject(Ljava/lang/Object;)Ljava/lang/Object; SharedXdsClientPoolProvider.java:102
    #8 io.grpc.xds.XdsClientFederationTest.cleanUp()V XdsClientFederationTest.java:86
```

xds/src/main/java/io/grpc/xds/ControlPlaneClient.java

@@ -199,6 +199,7 @@ boolean isInBackoff() {
     return rpcRetryTimer != null && rpcRetryTimer.isPending();
   }
 
+  // Must be synchronized.
   boolean isReady() {
     return adsStream != null && adsStream.call != null && adsStream.call.isReady();
   }
@@ -207,6 +208,7 @@ boolean isReady() {
    * Starts a timer for each requested resource that hasn't been responded to and
    * has been waiting for the channel to get ready.
    */
+  // Must be synchronized.
   void readyHandler() {
     if (!isReady()) {
       return;
@@ -325,7 +327,7 @@ final void sendDiscoveryRequest(XdsResourceType<?> type, Collection<String> reso
 
     @Override
     public void onReady() {
-      readyHandler();
+      syncContext.execute(ControlPlaneClient.this::readyHandler);
     }
 
     @Override