I don't know when that will happen. I think it's very difficult to mine bitcoin with a smartphone. Because the mining rig alone is very difficult and takes a long time. So what if done with a smartphone. I don't think that will happen. But we don't know what technology will be present in the future.
In the future, better Smartphones will be invented and they might even have the same computing power as a decent computer GPU. On the other hand, at that time, there will be even more people involved in mining and for bitcoin mining to be profitable, you need to have a much more powerful mining rig than what's required of today. They both are moving at a pace that doesn't allow Smartphones to mine bitcoins and have decent earnings.
In my own opinion i feel that smartphones are already being used for bitcoin mining, there's a website which I go to download movies , and when I visit with my phone they say my extra computing power is being used for crypto mining while i surf the website.
Quite unsure about that since websites where you can download movies make money from Advertisements and they are as clickbait-y as they can get. They will use FUD like that just to get people's attention and you will probably redirected in a website that tells you to download a software to 'keep you safe'. Please stop believing everything that you see on the net. Do your own research if you really want to know about that stuff.
... I think if there would be a demand for that, it will happen.
And I doubt that there will be a demand for that. Who needs to mine bitcoins on a specialized phone anyway? There's no point in making your mining device portable and it will probably be extra expensive. Pretty much not feasible. It will be so expensive that people will just say that it's better to use a GPU to mine bitcoins and that they don't really need it. It will just be a waste of time and resource for the manufacturer. The only ones who will be buying this new tech are those that are going to make a review on Youtube and get paid much more than what they spent on the device. Other than that, a regular bitcoin Joe will not be buying it.