[Android] Handle external protocol correctly

The external protocol is not handled well.
1. There is a new interface at content resource loader
   delegate, which is "HandleExternalProtocol", it must
   return false for the protocol we want to handle
   differently.
2. The method "shouldOverrideUrlLoading" in XWalkContentClient
   is not hooked into navigation interceptor, so the override
   in derived client is not invoked at all.

BUG=https://crosswalk-project.org/jira/browse/XWALK-150
Merge pull request #1076 from wang16/external_protocol

[Android] Handle external protocol correctly