Is it possible to create an HDMI input adapter for android?
You can get USB capture devices that will take HDMI video in and encode it to e.g. H264. You can get Android devices that can USB master. So then it's a matter of drivers. It looks like it's doable if you can choose the device and supply your own kernel: https://stackoverflow.com/questions/12334612/analog-video-capture-to-android-phone