Introduce the AndroidCellularNetworkSecuritySources group

Add a group containing a single, initially hidden dynamic safety source
that won't be vissible by default. The phone process will provide the
status of the source if the feature flags enable it to.

Test: Navigate to safety center and see the new entry isn't there
Test: Manually trigger a status update and see the new entry displayed
Bug: 308985417
Change-Id: I7cd66403e49a15a69ccb0f6947639d206843602b