Simplify NoPlayer Listeners & Forwarders

[jonreeve]

Problem

There are many duplications around all the types of listener. A listener interface for each, a forwarder for each, a "collection of listeners" for each (that has too much logic in it for just that; some track state, etc). The ultimate interface segregation of the listeners is a good aim, but it doesn't justify all this duplication.

Potential Solution

Unify many if not all of these forwarders and listener collections and so on, and simply handle the interface segregation at the outermost layer.

Impact

There's a great cognitive load in following all these listeners and forwarders and how they all interact and which one you're viewing at any moment because they all look so similar, and there's changes that have been made to one or two but not all, because of the duplication. Simplifying this need not change the outermost interface to this, but would make following and maintaining this code much easier.

[Mecharyry]

Version 2.6.0 of exo-player introduces the abstract class Player.DefaultEventListener which can be used over the more verbose Player.EventListener.