Observed in #5780.

1. Enable 'don't keep activities'
2. Start new post, upload media
3. While media is uploading, background the app (don't leave the editor)
4. Wait for upload to complete
5. The post is updated asynchronously, and the store copy of the store is updated
6. Re-open the app
7. The editor is recreated, but uses the PostModel it saved in onSaveInstanceState(), which doesn't have the updated post with the remote media URL

We should change EditPostActivity to try and fetch the latest copy from the PostStore if it's restoring a post from saved state.