Using a new thread to post to observers
You can limit the number of threads you're using via a thread pool. For example:

public class EventManager { 
    private static ExecutorService pool = Executors.newFixedThreadPool(8);
    [....]


and then use it like so:

pool.execute(new MyRunnable(this, event));


You should also take care of shutting down the pool when/if you dispose the EventManager.
For further information check the API docs (Android in your case).