SDK / exoplayer

- Support setting the user-agent in CronetDataSource
- Support setting the default user-agent in CronetEngineWrapper
- Use the underlying network stack's default user-agent by
  default. Many applications will configure the underlying
  CronetEngine or OkHttpClient with a user-agent that they
  expect to be used throughout their app, so always overriding
  this with our own default, on reflection, is not the best
  thing to do!

Issue: #8395
PiperOrigin-RevId: 350921963