[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.
[Extensions] Protect ExtensionService in XWalkBrowserMainParts

There is no need to expose XWalkExtensionService anymore.
This will also help us protecting the correct workflow
of registering extensions through XWalkExtensionService::Delegate.