Add custom ice stdnet implementation (#754)

On Android, because of the hard SELinux policies can not list the
interfaces of the ICE package. Without it can not generate a host type
candidate. In this pull request, the list of interfaces comes via the Java
interface.

client/internal/stdnet/iface_discover.go

@@ -0,0 +1,8 @@
+package stdnet
+
+// IFaceDiscover provide an option for external services (mobile)
+// to collect network interface information
+type IFaceDiscover interface {
+	// IFaces return with the description of the interfaces
+	IFaces() (string, error)
+}