HTTP auth for Chromium HTTP stack (Java side)

The existing Java callbacks become JNI callbacks. We splice this
functionality into HttpAuthHandler, splitting the existing callbacks
off into a new package-private class HttpAuthHandlerImpl. The public
API is unchanged.

Corresponding C++ change: https://android-git.corp.google.com/g/63763

Change-Id: Ia9717b86cbd2c32f17a2f8fad0cec34419c1376b