SDK / exoplayer

Update it to allow any listener class to be registered (and thus
require the caller of dispatch() to provide the type of listener
to call).

Maintain MediaSourceEventListener.EventDispatcher as a sub-class
for now so that all existing references continue to work. This
avoids creating a huge diff in one CL. The intention is to in-line
these incrementally.

This is pre-work for issue:#6765

PiperOrigin-RevId: 298818198