GetSubnetMask doesn't work for Android 7.0

[EbramTawfik]

I was debugging Android 7.0 to find why “GetSubnetMask” doesn’t work and I found that in method in android plugin return “null” and the reason is that it searching for IPv4 but “HostAddress” returned as IPv6 and that because Android 7.0 support IPv4 and IPv6 https://github.com/rdavisau/sockets-for-pcl/blob/dev/Sockets/Sockets.Plugin.Android/CommsInterfaceNative.cs#L41