utils: Make most FlagPack methods const
utils: Make most FlagPack methods const

Refs #74376

Change-Id: I869e3afd01a9aeab1ce5ce80f17eb71147875883

savoirfairelinux/ring-lrc,savoirfairelinux/ring-lrc,savoirfairelinux/ring-lrc,savoirfairelinux/ring-lrc,savoirfairelinux/ring-lrc