Improve offline detection logic

[arrdalan]

Currently, we use the app-triggered heartbeat to detect if the camera is offline. This has a few problems:

• App-tirggered heartbeat is not used frequently, hence there's a large delay for us to detect offline status.
• Sometimes, when this heartbeat is triggered in the background, network access is restricted and hence the heartbeat fails, further increasing the delay.

Instead, we can use a different scheme where the camera periodically (e.g., every minute) sends a heartbeat to the server. The server can then inform the app that the camera is offline if it doesn't receive a few consecutive heartbeats.