[TIMOB-26946] iOS: Launch screen shows last screen when quickly relaunching the app

To be reproducible easily on KitchenSink-v2: Launch the app, kill it from multitasking and quickly reopen it. On native apps, the launch screen is shown again. On Titanium, it shows a screenshot of the last screen visible. Most interesting: If you wait a few seconds, it's not showing the last screen and behaves correctly. I suspect this being related to the kroll-cleanup taking longer than expected, but if this could be investigates, we'd greatly appreciate it! *EDIT*: Busted! [This](https://github.com/appcelerator/titanium_mobile/blob/master/iphone/TitaniumKit/TitaniumKit/Sources/API/TiApp.m#L1014-L1016) will cause it. I suspect now that kroll-thread is fully removed, it could either be removed completely or be replaced by less harming dispatch-queues *EDIT 2*: Removing it indeed fixed the issue for our test-app. No crashes detected either. *EDIT 3*: The code was added 9 (!) years ago ([here](https://github.com/appcelerator/titanium_mobile/commit/cfa5485bc08af28fd6213b8f74fd681f6b43bfd2#diff-3808dfd4ad9381dd993eae7568b09ac4R341)) and was related to kroll stability. Filing a PR now.