No I don't think, that there is a function to do that. I would draw an additional polygon with the same coordinates and make a black fill color with a high transparancy, something like this code:
B4X:
Dim polygonoptions1 As PolygonOptions
polygonoptions1.Initialize
polygonoptions1.FillColor=Colors.ARGB(50,0,0,0)
polygonoptions1.AddPoints(sameListAsthePolyline)
PolygonDigit = GoogleMapsExtras1.AddPolygon(gmap, polygonoptions1)