The way CronetDataSource.Factory can internally fall back to another
factory is confusing and unnecessary. In the case the application cannot
instantiate a CronetEngine, it should directly use another factory.
This makes CronetDataSource more consistent with OkHttpDataSource. In
both cases, their factory constructors take the component that the
respective libraries advise should only be created once by the application,
which are CronetEngine and OkHttpClient respectively.
CronetEngineWrapper will be deprecated in a subsequent change.
PiperOrigin-RevId: 381016504
