SPARK-4843 [YARN] Squash ExecutorRunnableUtil and ExecutorRunnable

ExecutorRunnableUtil is a parent of ExecutorRunnable because of the yarn-alpha and yarn-stable split. Now that yarn-alpha is gone, this commit squashes the unnecessary hierarchy. The methods from ExecutorRunnableUtil are added as private.
Adding the type here is not really necessary (although it makes things more consistent). But I'd suggest just removing this variable altogether, and rename the constructor parameter instead).