[TIMOB-19425] Remove the need to call Ti.WatchSession.activate()

The Ti.WatchSession.activate() method has to be called before the Titanium can receive events from the Watch. This is how it works in a Xcode app as well. However, when you one of the Ti.WatchSession properties or methods it session is activated automatically. I'd suggest to automatically activate the session when the user adds event listeners to Ti.WatchSession as well so that there's no need anymore for Ti.WatchSession.activate(). The need to (sometimes) call activate() is something people easily forget and I expect it to cause Q&A question like _Why doesn't my Titanium app receive events from the Watch?_.