Able to manually control the activation and termination of IOSPlatform

As a part of application, the platform lifecyle is inconsistent with the app. We have to manually bring the platform active and terminate it in the main app. I added two methods activate() and terminate() in IOSPlatform and also nixed some imports.

I'd been trying not to add additional services on IOSPlatform. It seems that we have to do that for now. Thanks the Android architecture, it makes our integration much easier.