Remove Mockito from NitzStateMachineImplTest

Remove Mockito from NitzStateMachineImplTest in favor of fakes.  Mockito
was getting in the way more than helping. Instead, interfaces have been
extracted and two fakes created.

This commit contains various other refactorings to try to improve
readability and prepare for the next commit, which puts extra
requirements on the test support code.

Bug: 133492648
Test: atest com.android.internal.telephony.NitzStateMachineImplTest
Change-Id: I7595c81274e695c6d35a6ce508aed9bc4a0f62e6