Remove toString() for CarrierIdentifier.

We may get NPE if mCarrierIdentifier is null. + should convert it to
string by itself.

Test: test on phone
Merged-In: Ibd352d6d2a9b0cb33a494d108eb0d3a7c1dbc61b
Change-Id: Ibd352d6d2a9b0cb33a494d108eb0d3a7c1dbc61b