load live conversations properly

Gmail's provider generates a loadFinished for live conversations
once with zero messages, and then again once all messages are
fetched. Ignore zero-sized cursor updates, but only during
initial load. A previously rendered conversation that becomes
zero-sized is a signal that the conversation is no longer valid.

Bug: 6219354
Change-Id: I98c90da9c8f4e7d7249b779076f445315252ad5d
1 file changed