[Android]  Fix the http auth issue due to activity.

Currently, the http authentication dialog activity is
transforming the context to activity, which has side effect
on MixContext for the crosswalk package.  This fix is used to
correct the http authentication dialog activity.

BUG=https://crosswalk-project.org/jira/browse/XWALK-634
Merge pull request #1267 from ShDavidliu/http_activity

[Android]  Fix the http auth issue due to activity.