Add name space settings_stats

settings_stats name space contains whitelist keys which is needed
for logging.

Design doc: https://docs.google.com/document/d/1OijtO3QfFxYiIUyNbVm7pGzZ8zx2xV-1CrLYQEIGDTI/edit
Bug: 142941011
Test: rebuild

Change-Id: Ib6cb2b1203dbc0f5c575099e91dd0a40a3e17ca9