Fix "Invalid card" text on CarrierTextController

If there's an invalid sim that does not have a corresponding sub, the
text "Invalid card" should be appended to the current carrierText. As
this was assembled after this step (when it was empty), it ended up
replacing the correct carrierText instead of being appended.

Test: atest CarrierTextControllerTest
Bug: 138605164
Change-Id: Ibe0e2c3a218ea5c50f94e79cc46bd985b9646af0