Therefore it's plausible to me that the protrusion we see could be the top of a cell phone. In fact, it looks to me like it could be an iPhone 6 Plus. It's not conclusive to me that the protrusion is on the outside of the car.
I think you’re right about Jenner holding a cell phone. It’s clearly on the inside as the edge of the glass obscures the side of the phone.
Enlarging the photo, please note the edge/line of the glass window seemly being in front of/over the object in question.
OMHO
Your right.He’s holding a cell phone.