Fix white screen issue when application paused

onShow() and onHide() will be called by XWalkView internally, depends on activity lifecycle change.
Avoid calling these two methods directly, otherwise there will be a white screen displayed.

BUG=XWALK-1970

(cherry picked from commit d32a634699cb480d8d5b0869be3e7c5092936e56)
Merge pull request #134 from kurli/crosswalk-8-white-screen

Fix white screen issue when application paused