Adding missing synchronization in the runnable that is causing access…

… guard violation error. (grpc#11277)

binder/src/main/java/io/grpc/binder/internal/BinderTransport.java

@@ -327,7 +327,9 @@ final void shutdownInternal(Status shutdownStatus, boolean forceTerminate) {
                 inbound.closeAbnormal(shutdownStatus);
               }
             }
-            notifyTerminated();
+            synchronized (this) {
+              notifyTerminated();
+            } 
             releaseExecutors();
           });
     }