Reapply aosp/970247 to fix SubInfo#toString.

Looks like somewhere along the line this was partially overwritten by
another change. This makes the toString value consistent and stops the
MNC field from running into the country ISO code.

Test: compilation (trivial)
Change-Id: Ifdc00d340ae8ce31c224decebdd6c7aa4230bdb7
Bug: 8675309