Add flag and unit test for BarringInfo#getCellIdentity

This change enables clients to identify the specific network type for a given BarringInfo.
By exposing the CellIdentity, clients can more accurately apply access control rules (e.g., SSAC) that are relevant to the device's active network, leading to more robust and precise behavior.

Changes:
1. Feature Flag: The cell_identity_for_barring_info aconfig flag to control the API.
2. Unit Test: A unit test in BarringInfoTest to validate the core logic of associating a CellIdentity with BarringInfo.

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: I16715c1573efc69cf06a1d5139b37c785e2f49e3