SDK / exoplayer

* Remove casting getInstance and getCallback methods and just
  use member variables that have the more specific types. I
  didn't manage to get rid of casting completely; there are
  still one-off casts during construction. We should look at
  removing those in the future, but it's not completely
  trivial due to the way the Impl classes back-reference
  their wrapping classes.
* Move all callback invocations inside the Impl classes for
  consistency, and properly encapsulate the callbacks there.
  Sticking with the "OnHandler" naming convention for these
  methods, but we should probably tweak that in a subsequent
  change.
* Encapsulate MediaItemFiller in MediaSessionImpl.
* Some misc cleanup (e.g., converting anonymous inner classes
  to lambdas where possible).

PiperOrigin-RevId: 414401978