Add getMobileNetworkOperator to CellIdentityTdscdma

For consistency with CellIdentityLte, Wcdma, and Gsm
adding this function. It will aid in future refactors
to possibly create a common interface for these classes.

Bug: 111808089
Test: compilation
Change-Id: Ia6b21bcc368089d8346951ae9476e7eb12a05130