xds: cancelled=true on watch close in XdsDepManager

1fd29bc replaced cancelWatcher() with watcher.close(). But setting
cancelled was missing. Because the config update checks for shutdown,
the cancelled flag no longer avoids exceptions. But it seems best to
continue avoiding any processing after close to avoid surprises.

Author: ejona86

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

@@ -525,6 +525,7 @@ public void onResourceDoesNotExist(String resourceName) {
     }
 
     public void close() {
+      cancelled = true;
       xdsClient.cancelXdsResourceWatch(type, resourceName, this);
     }