In the end I managed successfully to connect to PC. The issue turned out to be in the NtlmPasswordAuthentication();
constructor.
So, instead of this:
String user = Constants.username + ":" + Constants.password;
NtlmPasswordAuthentication auth = new NtlmPasswordAuthentication(user);
I changed to this:
NtlmPasswordAuthentication auth = new NtlmPasswordAuthentication("",
Constants.username, Constants.password);
I don't know why, perhaps it's because of ":" special character, perhaps because of Android, but passing an empty domain name, the user name, and password separately to the constructor, solved the issue.