You don't even need a separate processor, grab one of the ARM SoCs with Big/Little cores, control the camera in bare-metal /hard real time in the little core, a

You don't even need a separate processor, grab one of the ARM SoCs with Big/Little cores, control the camera in bare-metal /hard real time in the little core, and keep Android on the big core(s). There's some low-level driver/hal type work to get that setup - but it isn't magic or even a particularly large investment of resources.