SDK / exoplayer

These constructors are currently very intertwined, passing `this`
references from the constructor of one to the constructor of another
before the first constructor is complete (and so the `this` reference
isn't really valid yet).

This change uses checker framework `@UnderInitialization` and
`@NotOnlyInitialized` annotations to make it more clear that the
references are not available yet. For the one 'direct' access needed
in the second constructor (calling `getApplicationLooper()`) we now
pass the `applicationLooper` directly alongside (to avoid needing to
dereference the reference 'too early').

This change also ensures that where a class hierarchy has a
'dependent' class hierarchy, the 'subclass' instance is always used
(by both subclass and superclass) without casting or manually hiding
the superclass field, by defining an overridable `getFoo()` method
instead and always using it.

#minor-release

PiperOrigin-RevId: 462335043