RESTRICT AUTOMERGE Update getDefaultCBRPackageName for fixing security vulnerability

Bug: 381419370
Test: atest CellBroadcastServiceTests
Test: Install fake cellbroadcast app. Check if cellbroadcast message is
received
Flag: EXEMPT bug fix

Change-Id: If2e797dd6d352627ddfac16f3d0447be0e172d25
(cherry picked from commit f24f40e2512f450b6a23536a4209318cd3043d99)
Merged-In: If2e797dd6d352627ddfac16f3d0447be0e172d25