Add application_foregrounded event for Android

[kanemara]

Currently this event is reported only by the Swift sdk, according to docs.
We use Segment as analytics provider in both iOS and Android apps. But such divergencies makes it impossible to have all Mobile analytics combined and analyzed seamlesly using same metrics.
This event is common for mobile platforms, so making it exist for all looks logical and convenient.

[lhunath]

For more details, see segmentio/analytics-android#847.
Also, the rationale is described in segmentio/analytics-swift#172 where it is already accepted and implemented.

However, please be aware of issues with the current Swift implementation: segmentio/analytics-swift#387, as well as the strange definition of the opened event.

I believe Segment will need to be clear about what the definition of opened and backgrounded are before determining what foregrounded should mean.

I actually suspect that the current definitions are asymmetric and misleading (opened = foregrounded?).

Segment might want to rename/copy the existing opened event to foregrounded; or deprecate the existing events and create new ones with clear & symmetric definitions, such as:

• application_launched [to_background]
• application_activated [from_background]
• application_deactivated [to_background]
• application_terminated