Support duration and durationInForeground on macOS

[kstenerud]

Goal

Currently, the duration and durationInForeground fields are always zero on macos because we don't record that information. This PR makes macOS start recording this information so that users can see it in their reports.

Design

MacOS does record foreground/background state changes, but uses notification names that are confusing when compared to ios. However, they serve purposes that are close enough to be useful for our needs.

MacOS:

Notification	Meaning
NSApplicationDidBecomeActiveNotification	Application is in foreground
NSApplicationDidResignActiveNotification	Application is in background
NSApplicationWillTerminateNotification	Application is shutting down

iOS:

Notification	Meaning
UIApplicationDidBecomeActiveNotification	Application is active
UIApplicationWillResignActiveNotification	Application is interrupted
UIApplicationDidEnterBackgroundNotification	Application is in background
UIApplicationWillEnterForegroundNotification	Application is in foreground
UIApplicationWillTerminateNotification	Application is shutting down

The approach here simply uses the macOS equivalents to trigger the same handler code as for iOS, allowing existing handlers to do the calculations as they have always done.

Changeset

Added compile-time-conditional code to add notification handlers for macOS to handle foreground/background and app termination.

Testing

E2e tests don't exist yet for macos, so I manually triggered events in the macos test apps and verified that durations were recorded on the backend.