Remove hidden connectivity methods reference in ApnContextTest

NetworkConfig is hidden connectivity class. It's now not
accessible outside the module. It's actually not used, so remove
it from the test.

Bug: 182859030
Test: atest ApnContextTest
Change-Id: If804d524613d451a578ef0a06defd95054ce0aeb
Merged-In: If804d524613d451a578ef0a06defd95054ce0aeb