Update the Test to Test CellIdentity Redaction

Make use of a hidden method to more correctly unit test
that the location-sensitive info is being redacted from
the BarringInfo. This allows the test to be decoupled
from the specific implementation of CellIdentity#toString()

Bug: 361387150
Test: atest TelephonyRegistryTest #(test-only change)
Flag: EXEMPT bugfix
Change-Id: I002679a2dde4690003dd1a2862bccb9ccfcd122e