Offer a callback to intercept url requests.

The new callback allows applications to return a response for a particular url.
Deprecate onLoadResource in favor of the new api.  onLoadResource is not
currently being used by the new network stack and applications can post a
message from shouldInterceptRequest to handle onLoadResource.

Bug: 2905943
Change-Id: Icf48ab3110d85d76112b3c3b4a1a8c8a333b82f0