[PLAT-6134] Fix inaccurate app.duration values

[nickdowell]

Goal

Fix the calculation of app.duration and app.durationInForeground when multiple calls to notify() are made.

Changeset

The root cause was that each call to bsg_kscrashstate_updateDurationStats() was incrementing the duration by adding the time elapsed since appStateTransitionTime.

The fix is to make sure each call to bsg_kscrashstate_updateDurationStats() only adds the time elapsed since it was last called.

To make this clearer, appStateTransitionTime has been renamed to lastUpdateDurationsTime.

The accuracy has also been improved - the values were previously being rounded to the nearest second.

Testing

Added an E2E scenario that can replicate the original issue, and verified that it now passes.

[github-actions]

Infer: No issues found 🎉

OCLint: No issues found 🎉

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