Do not volunteer this fact.
If they ask, "When are you available to start?", use your discretion. On the one hand, you could say, "Immediately." But I think it would be best to remain calm, and act as if you're still employed and you'll need two weeks past the day of the offer letter. If you receive an offer, you could certainly reveal more, and say something to the effect of, "Things have actually slowed down in the past few days and I may be available even sooner if it would be best for the company."
Note: I'm not necessarily encouraging you to lie. After all, they may ask, "Why are you looking for a new job?" And if they do, you will need to give an answer. At this time, you may say something to the effect of, "My current employer is experiencing financial hardship." And if you eventually do receive an offer, the HR department may ask about your employment history. If they do, then I recommend complete honesty because they can verify these dates with your former employer(s).
But I am advising you to be less than forthcoming about the fact that your employment status changed after you applied for the job. If they know this, then it may compromise your competitive standing. They may (perhaps erroneously) assume that you were laid off because of poor performance. Or they may use this fact to their advantage when negotiating a wage/salary.
So to summarize, if you are asked for dates of employment: Tell the complete, unvarnished truth. But in light of the fact that you were laid off after you applied for this job, I see no reason to volunteer this fact.