Clear cached NITZ data in a few more places

Clear cached NITZ data in a few more places where the network becomes
"detached".

1) ServiceStateTracker.mLastNitzData (which should have been done in
commit b2bc7b71)

2) Introduce calls to NitzStateMachine.handleNetworkUnavailable()
and make NitzStateMachine do less work if nothing about the NITZ state
has actually changed (to guard against too many calls overall).

3) Stop calling NitzStateMachine.handleCountryUnavailable() - this
should only be called by the LocaleTracker, and these calls were
probably originally intended to clear the last NITZ data received but
has been obscured by the many changes that have happened in the
LocaleTracker / NitzStateMachine code over the last couple of years.

Bug: 140712361
Test: treehugger
Change-Id: I2be2ceccb21494c7193def56b7f14e2ca205b5fa