Fix race condition in AsyncSecurityPoliciesTest.

Synchronized with the executor before asserting cancellation of the
delegate future to ensure that transformAsync has finished processing
the delegate future and propagated the cancellation.

TAG=agy
CONV=e1bfa5a2-e855-4f79-abdd-ef2b264977be

Author: kannanjgithub

binder/src/test/java/io/grpc/binder/AsyncSecurityPoliciesTest.java

@@ -289,6 +289,7 @@ public ListenableFuture<Status> checkAuthorizationAsync(int uid) {
     ListenableFuture<Status> authFuture = asyncPolicy.checkAuthorizationAsync(SOME_UID);
     assertThat(awaitResult(settableUid)).isEqualTo(SOME_UID);
     authFuture.cancel(false);
+    executor.submit(() -> {}).get(10, TimeUnit.SECONDS);
 
     assertThat(delegateAuthFuture.isCancelled()).isTrue();
   }