[SystemInfo]: Fix crash in wifi network module on desktop.

This commit fixes the deletion of pointer owning by GVariant.
[SystemInfo] : Initialize the plugin instance only when need.

Moved plugin instance initialization from CreateExtension() to CreateInstance().

XW_Initialize/CreateExtension is intended to call by the xwalk extension server
while registering the extension with the server. CreateInstance() is called
when exactly the api is accessed by application, so this is the point where we
need to initialize extension.
Merge pull request #275 from avalluri/master

SystemInfo extension module fixes