Fix timing issues when Bugsnag started after the focus event

[JamesLindsay0]

Goal

Fix issues where Bugsnag is started after the focus event is received (so the foreground timer was never started).

Changeset

• Start the foreground stopwatch automatically on startup.
• Use the Duration rather than the ForegroundDuration to determine if the app has launched as it takes account of the time before Bugsnag was started.