Fixed concatenation to properly discard empty strings

Before, concatenation of carrier names was done naively. Now we properly
discard empty strings so there are no dangling separators.

Extra mocking added because of temporary change in
CarrierTextController. These should be removed with b/130246708.

Test: atest
Fixes: 130363633
Change-Id: I17bd437a4789eb48b3e4661f294c64e14efc9261