Remove FIXME that getApnSetting should be NonNull

API Council has asked that I remove a comment suggesting that
this API be NonNull. While in theory, we should be able to
make this NonNull in the future, the tooling doesn't work in a
way that makes such a change beneficial.

Bug: 151928851
Test: compilation (comment-only change)
Change-Id: Ifd80f2859e4a090225aab1b3a483325d654a2412