Frameworks/base: Make debug helper lazy in ConnectivityManager

Place helper map for callback field names into a holder. This avoids
the reflective calls in static initialization of ConnectivityManager
in the common (non-debug) case, which means the class can be
compile-time initialized. Also saves the storage necessary (both
the storage array as well as the reflection metadata).

Follow-up to fcfa7d92.

Bug: 27265238
Change-Id: Ib4bfaf27acd234a035a5d198458340099a156a4c