[Android] Fix the issue onPageFinished callback would be called twice

This commit contains the following changes:

o Only rely on didStopLoading to call onPageFinished callback. Generally, both
  didStopLoading and didFinishLoad callbacks are the same except
  1) didStopLoading will also be called for extended protocols, like mailto:,
     tel: etc;
  2) didStopLoading will be called if user clicks "X" button on the browser UI.

o Loading javascript: URL should not cause onPageFinished calling since it is
  assumed that javascript: url is executed in an existing JS context in a web
  page already loaded.

o Port some test cases from android_webview/ for onPageFinished test.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1017
TEST=org.xwalk.core.xwview.test.XWalkClientOnPageFinishedTest