Improve app start integration

[buenaflor]

Description

Currently we use completers to wait for multiple things to finish before proceeding with the app start, this is problematic since it might block the event processing if for some reason the completer never finishes.

Things to change

• move app start ttid from navigator observer to app start integration (so root ttids are ignored in observer)
• thus app start integration is now the source of truth where app start happen
• handle case where addPostFrameCallback within the integration might not return, for example if SentryFlutter.init is called within an isolate
• deprecate setAppStartEnd

Fixes:

• Transactions are very slow to complete when started inside appRunner, block the UI from showing up #2233
• pendingTimer when testing with SentryNavigatorObserver on sentry_flutter 8.2.0 #2103