This also throws away the logic of updating
`API.LocalUser.Value.Statistics`. Components should rely on
`LocalUserStatisticsProvider` instead for proper behaviour and ability
to update on statistics updates.
- make `UserActivity.InGame` and derive that to `InSoloGame` and `InMultiplayerGame`
- rename `SoloGame` to `InSoloGame`
- rename `MultiplayerGame` to `InMultiplayerGame`