[Bug] Nearby users duplicated

[pawisoon]

When running example code on 2 physical devices, and leaving the app for a moment to background and re-entering it, the list of nearby users contains 2 of the same users.

[gaedokan]

The devices are duplicated because Multipeer Connectivity cannot function while the application is in the background. To address this, all MCSession instances and peers should be disconnected and torn down when the app is backgrounded