Minor cleanup in AtmosphereBehavior

Cache call results if they are used more than once

(cherry picked from commit 353aa4e8b449a5df1b2b1f97935ca450cdc348fb)
Make members final when there is no need to modify them later

(cherry picked from commit a969f5999d9b25c867ff0dc904186cd66de6b76b)
EventBus#removeRegistrationListener() should *remove* the listeners

(cherry picked from commit c539f680b0be205322dfbec2cb1281e473d22c12)
Cache the got value and reuse it in a method

(cherry picked from commit ca6e424c89d3b75216e58bf0733637aeb37c4a8c)
Use #getBroadcaster() to be able to use a custom one for testing

(cherry picked from commit d0a29b0e54b2cd474c01637fe278887c5351ada7)