Loading indicator does not go away when the event queue appears to be active

[PIG208]

We added a loading indicator in #852 that appears when even queue polling fails with any error.

However, it seems that the loading indicator can stay on even the event queue is doing fine, and does not go away until you get a new event.

I don't have a consistent way to reproduce this, but what I did was basically:

1. Leave the app on and lock the phone
2. After a while, unlock the phone
3. Observe if the loading indicator stays there longer than usual
4. Try to do something (like starring a message), and the loading indicator should go away promptly

My guess is that when the event queue goes stale, isLoading gets set to true. When we do get the new event queue, we do not set isLoading back to false.