Remove usage of setDefaultImpl in test

This is the only usage of setDefaultImpl on the interface. Removing this
usage allows not generating all the code managing default
implementations in the aidl interface, as it will soon not be generated
by default.

Bug: 206006915
Test: atest NetworkStackServiceTest
Change-Id: I969740122eb4496a919678f94a4eb1848b6b2b16