* ActivityLifecycleIntegration:
- creates `onCreate` and `onStart` TimeSpans
- set app start type to warm in AppStartMetrics when needed
- saves SystemClock.uptimeMillis to set app start timestamp
- sets start type to warm even when cold start was invalid (app was started in background, like via BroadcastReceiver)
- restart app start in AppStartMetrics in perfv1, too

* reverted TimeSpan.setStartUnixTimeMs to @testonly method
* AppStartMetrics has now a method to restart appStartSpan and reset its uptime_ms
* PerformanceAndroidEventProcessor now attaches activity start spans to warm starts, too
* SentryPerformanceProvider doesn't create spans anymore
* TimeSpan.setStartUnixTimeMs now shifts other timestamps accordingly