Refactor app wakeup to use DidBecomeActive instead WillEnterForeground

[wutschel]

As discussed in #1170 the app's wakeup shall be reworked to use applicationDidBecomeActive instead of applicationWillEnterForeground. This is required to make sure the socket connection is back (which is currently done via a timeout). This needs some additional logic as applicationDidBecomeActive is not only called during app's wakeup but also during normal startups.