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

The right behavior is onPageFinished callback of XWalkClient should only
be called one time when it completes to load the main frame. This commit
removes a duplicated invokation of onPageFinished, and also ports some
tests for onPageFinished callback from android_webview/.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1017
TEST=org.xwalk.core.xwview.test.XWalkClientOnPageFinishedTest