Cleanup the background task that pauses the SDK when resigning active.

[pixlwave]

In the AppCoordinator we currently wrap the call to pause in a background task. However we never call stop() on this task which leaves it running, resulting in the system terminating the app. Steps to fix:

• Investigate whether Rust actually needs more than a few seconds to pause (the task is there to extend this time).
• If it does, then either the ClientProxy.stopSync() method should be made into an async/blocking call, or we need to add a callback so we can stop the background task when it is finished.