Revert "[Settings] Remove CarrierConfigManager.Apn.KEY_PREFIX from API"

This reverts commit 72f1472aaca88045078b6b296ddae2819377a339.

Reason for revert: b/174645163#6

Bug: 174645163
Test: build passed
Change-Id: Ied3c6af8b9d03629e3abd377ad032d9e4bb35fd8
2 files changed