I assume this is a still frame from a video, because 16:9 is a common video ratio. In order to maintain constant frame size, "digital zooming" needs to be reinterpolated. Thus, yes, you need to account for the "zoom ratio" by factoring in 1.4 times the focal length.

However, your calculations are off, because a "1/3.1-inch" sensor is not actually 1/3.1" in dimension. This is understandably very confusing, but it is merely a nomenclature referring to old circular 1" video tubes, which had a useful diagonal of about 16 mm, far from the actual 25.4 mm in an actual inch. See also, Why is a 1" sensor actually 13.2 × 8.8mm?

It took a little bit of Google searching, but I found the Sony Experia XA (Sony F3115) has a Sony EXMOR IMX258 sensor. Wikipedia's EXMOR article says the sensor is a 1/3.06" format, with a diagonal of 5.867 mm, and a pixel pitch of 1.12 µm. At 3.5 mm (the lens focal length), a 1.12 µm pixel subtends atan(0.00112 / (3.5 * 1.4)) = 0.0131°. Or if you prefer, 1/0.0131 = 76.4°/pixel at the center of the image. This distinction is important because near the edges, a 1.12 µm distance subtends a smaller angle — this is the nature of the tangent function.

A 400-pixel region subtends an arc of 2 * atan((0.00112 * 400 / 2) / (3.5 * 1.4)) = 5.23°

Normally, a very useful reference is Wikipedia's table of sensor format and sizes. Unfortunately, it doesn't list a 1/3.06" sensor. It does list a 1/3.09" Sony EXMOR IMX351 sensor, which is fairly close though.