Fix state clearing in (old) NitzStateMachine impl

This commit makes sure that NitzStateMachineImpl.mSavedTimeZoneId
is cleared when a time zone cannot be determined. This value controls
the behavior when the user toggles auto time zone off and on; without
it being cleared the state machine remains "opinionated".

Bug: 140712361
Test: atest com.android.internal.telephony.NitzStateMachineImplTest
Change-Id: I39a78d83fe516ed46abb26be257ba7af2ee0926a