Revert "Switch from custom K_D to Q_DECLARE_PRIVATE_D & Q_D"

Q_DECLARE_PRIVATE_D uses reinterpret_cast with the d member in the
defined methods, which results in bad casting with the multi-inheritance
subclasses of ServiceBasePrivate.

Switch to use KDNSSD_D instead of K_D to reduce risk of potential clashes.

This reverts commit e8f08295.

BUG: 432949