[Runtime] Fix application extension registration issue

ISSUE: https://crosswalk-project.org/jira/browse/XWALK-759

When launched as an external web browser, the application extension instance
will be created and trigger "CHECK(application)" assertion failure if
xwalk.app.* APIs are called.

To fix this, now we only register the application extensions after check whether
we have a currently running application in ApplicationService.
Merge pull request #1445 from seanlong/crosswalk-3

[Runtime] Fix application extension registration issue