[API Feedback] Rename getChannelNumber to getNrarfcn

For consistency with the rest of the public API, the CellIdentityNr structure
should use the specific term for the channel number that is returned
(NR-ARFCN) in the API name rather than the generic getChannelNumber().

Bug: 125025882
Test: build + cts + unit test
Change-Id: Id5bc3e6f9595922540adebc3e574227e88b029ce