Move GsmSmsCbMessage to CBReceiver test app

After removing platform handling of cell broadcasts (in favor of
CellBroadcastService module) the GsmSmsCbMessage class is only used by
CellBroadcastReceiver test app, so move the class there.

Bug: 135956699
Test: manual
Merged-In: Ia1d19356c34f9a13638c10d1c1b9e53c6b69fddd
Change-Id: Ia1d19356c34f9a13638c10d1c1b9e53c6b69fddd
(cherry picked from commit 803c8855)