Revert "Expose NetworkRegistrationInfo.getNrState() as public API"

getNrState() was exposed. However, it doesn't actually get the NR
state in most cases and it is a mixture of a couple different
concepts. This API does not provide the correct 5G NR state, so
revert it from API surface and also the related change.

Bug: 171183530
Test: make update-api ; m
Change-Id: I651ed14500be5af45dd896ae5da1da36ad249a26