Add missing `application_foregrounded`

[lhunath]

Akin to the rationale provided in the following task: https://github.com/segmentio/analytics-swift/issues/172

We should add an application_foregrounded event to bookend the current application_backgrounded event.

The fact that this event now exists on iOS gives additional weight to this argument, since we now end up with iOS/Android applications which log application_foregrounded events only for iOS, omitting Android, resulting in skewed data analytics perceptions regarding platform-based usage patterns.

Ideally such standardized events should be cross-platform managed and adding a new standard event to one platform should ensure that this event is equally exposed on other platform SDKs to ensure consistency across the SDK and avoid such situations in the future. The platform SDKs should not be allowed to diverge arbitrarily.