Add default constructor to NR cell classes

The default constructor is missing in CellInfoNr and CellIdentityNr
classes. All the other CellInfo* and CellIdentity* classes have it.

Also, add setCellIdentity(CellIdentityNr) method to CellInfoNr.

Bug: 147260058
Test: m
Change-Id: Iefa1ec346bcef612f020f79cbc858649105cd406