Add missing UARFCN Getter to CellIdentityTdscdma

Add a method to get the UARFCN for CellIdentityTdscdma.
There is an equivalent method in all the other classes,
so this is simply correcting an oversight.

Bug: 123957505
Test: atest CellIdentityTdscdmaTest
Change-Id: I743d90b4532b86edb34d267ae55eca73de2a0086