Expose BarringInfo#getCellIdentity as a SystemApi

This CL makes the BarringInfo#getCellIdentity() feature available to clients. By enabling clients to identify the specific network type for a given BarringInfo, they can more accurately apply access control rules (e.g., SSAC), leading to more robust and precise network behavior.

Changes:
Exposes the getCellIdentity() method as a flagged @SystemApi.

This change only handles the API visibility.
The corresponding feature flag and unit tests are introduced in a companion CL.

Bug: 431881770
Bug: 412452528
Test: atest BarringInfoTest
Test: atest TelephonyCallbackTest, atest PhoneStateListenerTest
Flag: com.android.internal.telephony.flags.cell_identity_for_barring_info

Change-Id: Idc3e7830d084f885201bcc47664d2d18d07300f7