Fix that cordova activity uses XWalk API with null pointer check

In some cases, XWalkView will fail to initialize, e.g. CPU
architecture mismatch detected. Need to check null pointer
to keep Cordova App from crash in such case.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1616
Merge pull request #109 from wang16/master

Fix that cordova activity uses XWalk API with null pointer check