SDK / exoplayer

The ForwardingPlayer implements the Player interface and forwards all
operations to another Player instance. Apps will be able to override
methods of ForwardinPlayer in order to modify and/or suppress specific
Player funcionalities.

This commit introduces the ForwardingPlayer which simply forwards all
Player operations to another Player instance. In follow-up changes,
the ForwardingPlayer will be extended so that it eventually reaches the
feature-set offered by ControlDispatcher.

PiperOrigin-RevId: 370653167