[Runtime] Enable to update a web application from local

A web application can be updated from local when using "--install" switch
in command line, like this:
$ xwalk --install /path/to/xpk_package.xpk
If the application has been installed, WRT will try to update it.

https://crosswalk-project.org/jira/browse/XWALK-463
[Runtime] Re-factor for Install & Update methods in ApplicationService class
Merge pull request #1396 from takethathe/update

[Runtime] Enable to update a web application from local