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

(cherry picked from commit f24f40e2512f450b6a23536a4209318cd3043d99)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:6266ceedbfaf435866455e9f9242dba3d9ae7309)
Merged-In: If2e797dd6d352627ddfac16f3d0447be0e172d25
Change-Id: If2e797dd6d352627ddfac16f3d0447be0e172d25