gfe-relnote: (n/a) Refactor Bbr2ModeBase::Enter and Bbr2ModeBase::Leave to allow BBR2 to switch modes outside the context of a congestion event. Refactor only, no behavior change. The intention is to switch modes after quiescence, for b/150230091. PiperOrigin-RevId: 298391824 Change-Id: Ib2e51d98111539d394da744cbc82da833e030ef1