fix(java-app-template): invoke app.run() (#2300)

Since the java client for jsii does not cleanly shutdown
the jsii-runtime process, the implicit "app.run()" does not
work, and the app cannot be synthesized.

Fixes #2289

Related: aws/jsii#456

packages/aws-cdk/lib/init-templates/app/java/src/main/java/com/myorg/HelloApp.java

@@ -10,5 +10,8 @@ public static void main(final String argv[]) {
 
         new HelloStack(app, "hello-cdk-1");
         new HelloStack(app, "hello-cdk-2");
+
+        // required until https://github.com/awslabs/jsii/issues/456 is resolved
+        app.run();
     }
 }