[Extensions][Android] Make CreateExtension call XWalkExtensionService directly

XWalkBrowserMainParts is now a XWalkExtensionService::Delegate, meaning that
XWalkExtensionService will call RegisterInternalExtensionsInServer everytime
a new in_process XWalkExtensionServer is created (for a new RenderProcess).

Therefore, XWalkExtensionService::RegisterExtension() has been removed
and XWalkExtensionAndroid will now need to register its extensions on
XWalkBrowserMainParts so them can get correctly registered on-demand
by XWalkExtensionService.