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 7e28683228.
Bug: 27265238
Change-Id: Ib4bfaf27acd234a035a5d198458340099a156a4c
9537278b82