[TIMOB-19603] Unified application lifecycle callbacks across platforms

As a developer, I want to use the same application lifecycle handlers, regardless of which platform I'm deploying to. On iOS the application is notified about its lifecycle via "pause", "resume" and "resumed" events from Ti.App. On Android, one has to to assign handlers to "$.win.activity" or (slightly differently) "Ti.Android.currentActivity". Since the iOS functionality doesn't have any indication that it's iOS-specific, It would make for much clearer code if Android used the same mechanism. This would probably be a breaking change but adaptation should be straightforward for any app that already supports iOS.