JDK 16 RFR of JDK-8250244: Address reliance on default constructors in java.net

[Joe Darcy]

Hello,

Please review the replacement of default constructors in various 
abstract classes in java.net with explicit constructors:

    webrev: http://cr.openjdk.java.net/~darcy/8250244.0/
    CSR: https://bugs.openjdk.java.net/browse/JDK-8250245

(This is part of a larger effort to remove default constructor usage 
from the JDK in preparation for enabling a new lint warning.)

Thanks,

-Joe