Corrects broadcast area for geo-fencing trigger message

We should use the union of the broadcast area for all cell broadcast
when it's not empty.

Bug: 141177480
Test: build & manual test
Merged-In: I742990eb6f00472c94b4afab8a74a5e113d235a7
Change-Id: I742990eb6f00472c94b4afab8a74a5e113d235a7
(cherry picked from commit 7be2041d)