I am using version 100.4 of the runtime.
It seems that the SketchEditor's default double-click to finish behavior is different between Android and the other platforms. When sketching a line or polygon on Windows or iOS, the point that is double-clicked to finish the sketch will be part of the final geometry. However, on Android the point that is double-clicked will not be part of the geometry, only the points that we're added already. Here is how I am using the SketchEditor:
await mapView.SketchEditor.StartAsync(SketchCreationMode.Polyline, false);