[AT&T] Add key to show 4g for 3g RAT.

This key is introduced for AT&T

This will also require a carrier-config side change
to set show_4g_for_3g_data_icon_bool to true.

This will replace H/H+ that is shown by AT&T today.

Bug: 138655095
Test: manually

Change-Id: I480e14be41f8edbe607472351efd39d6d91b81b0