Clear NITZ state when entering airplane mode

Clear NITZ time / time zone detection state when entering airplane mode.

This is to avoid an issue when users do the following:
1) Get on a flight
2) Set device to airplane mode
3) Turn off auto time zone detection
4) Manually set the device time zone to the destination time zone
5) Turn off airplane mode
6) Turn on auto time zone detection

The device would switch to the time zone of the departure point because it
retained state from before the flight, which caused annoyance for users.

Bug: 133492648
Test: atest com.android.internal.telephony.NitzStateMachineImplTest
Change-Id: I1266a53bf14097c72b9f122725ebbd75c439b05a