Every popular mobile OS has standard libraries for making HTTP requests and doing crypto.
I can forgive games for sacrificing size in favor of high-res animations and a blazing fast graphics stack, but I don't think LinkedIn, for example, is in need of particularly fast graphics.
If you've had to look at a pcap from what the android standard libraries do for https chunked upload, you would cry. One tls packet for the per chunk header, one for the chunk of data, and one for the carriage return newline. I've thankfully forgotten if these managed to also be separate tcp packets.
Given that, I don't think it's unreasonable to consider using a library to do it better, although you have to weigh the increase in apk size. The standard crypto libraries on the phone are great if the features you need were released 5 years ago, but if you want consistent behavior across Android versions and manufacturers, you need to include that yourself too.
Except the standard library will be updated if this actually became a problem, whereas you might choose that the cost/benefet ratio to updating your library is not worth it.
I can forgive games for sacrificing size in favor of high-res animations and a blazing fast graphics stack, but I don't think LinkedIn, for example, is in need of particularly fast graphics.