[Extensions] Disable the Extension Framework entirely for --install and --uninstall

As mentioned in https://github.com/crosswalk-project/crosswalk/issues/839,
we shouldn't be bootstrapping the entire BrowserProcess workflow when using
xwalk --install or xwalk --uninstall.

This patch disables the ExtensionService creation, the Extensions
loading and the Extensions registering as a workaround for the
crash mentioned in https://crosswalk-project.org/jira/browse/XWALK-31 .