Report Fatal App Hangs

[philipphofmann]

Description

When an app hang leads to a watchdog termination, we can report this as a fatal app hang. When the SDK detects a fully blocking app hang, it needs to store the app hang to disk and wait for the app to recover from the app hang. When the app doesn't recover, the SDK must send the app hang as a fatal on the next app launch. When the app recovers from the app hang, then it must send it as a fully blocking app hang. The downside of this approach is that app hangs will be reported a bit delayed, which is acceptable.

This issue is a prerequisite to #3319, because the app hang rate only considers fatal app hangs.