Expose KEY_DATA_SWITCH_VALIDATION_MIN_INTERVAL_MILLIS_LONG as public API

Rename from KEY_DATA_SWITCH_VALIDATION_MIN_GAP_LONG to follow the
canonical naming format. Expose as public API to avoid clients
depending on hide API.

The carrier config has been introduced since Android QT release
and is stable for now. Carriers like GoogleFi has overridden
the carrier config to take advantage of the feature for years.

A renamed carrier config key which aliases old key but shares old
value is created to rename the carrier config and keep the maximum
backward compatibility.

Bug: 181813215
Test: m -j offline-sdk-docs
Change-Id: Ibc60b1872c9d1c3db0e4f42776a2616339154370