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.

Original change: https://android-review.googlesource.com/c/platform/frameworks/base/+/1202369
Bug: 147260058
Test: m
Merged-In: I4617272446ffb32d5d33d14d825f4ed919cc1c47
Change-Id: I4617272446ffb32d5d33d14d825f4ed919cc1c47
(cherry picked from commit 23382679)