Add @experimentalapi annotation to triggerEvent and onEvent (issue #12979)

Author: kannanjgithub

api/src/main/java/io/grpc/ServerCall.java

@@ -110,6 +110,7 @@ public void onReady() {}
      *
      * @param event the triggered event.
      */
+    @ExperimentalApi("https://github.com/grpc/grpc-java/issues/12979")
     public void onEvent(Object event) {
       // Default no-op
     }
@@ -284,6 +285,7 @@ public String getAuthority() {
    *
    * @param event the event to trigger.
    */
+  @ExperimentalApi("https://github.com/grpc/grpc-java/issues/12979")
   public void triggerEvent(Object event) {
     // Default no-op
   }

inprocess/src/main/java/io/grpc/inprocess/InProcessTransport.java

@@ -739,7 +739,7 @@ private synchronized void setListener(ServerStreamListener listener) {
 
       void triggerServerEvent(final Object event) {
         synchronized (this) {
-          if (!closed && serverStreamListener != null) {
+          if (!closed) {
             syncContext.executeLater(new Runnable() {
               @Override
               public void run() {