Scheduler which uses handler to execute actions.AndroidSchedulers.mainThread() or null if default
should be used.RxAndroidSchedulersHook to use based on order of
precedence as defined in the RxAndroidPlugins class header.Scheduler backed by a Handler.Scheduler which executes actions on the Android UI thread.RxAndroidSchedulersHook implementation as a global override of any
injected or default implementations.register methods in this class
default implementation