Add the ability to disable real time updates in background mode

[matthewelwell]

There are several considerations in an Android application which mean that it would be beneficial to disable real time when the application is in the background:

• Battery usage
• Additional API calls to the Flagsmith API, unnecessarily consuming requests against the limit

Also, when the application is in the background and network requests are attempted, they are not necessarily guaranteed to complete which could end up in unnecessary exceptions being thrown.