Support CellInfoNr in LocaleTracker.

LocaleTracker has functions to get MCC/MNC from CellInfo. With later
added CellInfoNr, the orignal switch/cast style logic will miss the
support of CellInfoNr.

As CellInfo/CellIdentity already designed to return right MCC/MNC
strings (null for CDMA and meaningful one for others), we can directly
call getMccString/getMncString from CellIdentify to get MCC/MNC strings.

Bug: 152707330
Test: atest com.android.internal.telephony.LocaleTrackerTest
Test: atest com.android.internal.telephony.CellIdentityTest
Change-Id: If2aecd500e945c48b443ae9e7841cc8945603fde