Tablet-based editing software like Snapspeed and Perfectly Clear 3 lets you quickly fix images anywhere
Nik makes the b&w plug-in Silver Efex Pro, so it has plenty of monochrome know-how in its pocket. Snapseed is a much less expensive way to get access to it. When going grayscale, begin by tapping on the Presets button to see some options for starting points. Select the Film preset to use one that has grain already added in. As in the Selective Adjust tool, swipe up or down to choose to adjust Brightness, Contrast, or Grain, and swipe left or right to decrease or increase the strength of each.