Phone number only available for admin users

The phone number is hidden to non-admin users, following the same
reasoning that exists for other telephony-specific fields like the
baseband IMEI.

Bug: 392808943
Flag: EXEMPT bugfix
Test: atest PhoneNumberPreferenceControllerTest
Test: atest MobileNetworkPhoneNumberPreferenceControllerTest

Change-Id: I4e612219d0c7439930e91b3e1d6e368a0dfd073e