[Tizen] Display the top window when resume application from background.

For XPK applications with multipages, whenever a new page(window) is
created, it is saved as a root window inside current Crosswalk. In other
words, a list of root windows are created when running such kind application.

While Tizen app framework resumes the root window with stacking order
from bottommost(first) to topmost(last). As a result, the first page is
always showed when resume from background.

This PR provides only one root window per application, and all application
pages(windows) belong to this root window.

BUG = https://crosswalk-project.org/jira/browse/XWALK-900