[Android] Remove some of the duplicated code for handling Asset URLs.

The code paths in RuntimeURLRequestContextGetter::GetURLRequestConext() can be
greatly simplified if we stop using a separate job factory and a single-item
vector and just add a ProtocolInterceptJobFactory when on Android for the
asset:// protocol handler.

While here, we can also remove XWalkUrlRequestJobFactory, whose special 
functionality is not being used anywhere (if it ever was).