[Encode][PDVT-SH] Refactor AVC minimum QP handling across platforms

* [Encode] [PDVT-SH]Refactor AVC minimum QP handling across platforms
Refactored minimum QP value handling for AVC encoding by introducing a virtual GetMinAvcQp() method in the base class. This replaces platform-specific UpdateMinMaxQp() overrides with a cleaner design pattern. Base class returns QP=10 by default. Platform-specific implementations override to return QP=5 for VBR rate control on certain platforms, and QP=1 for all rate control methods on the latest platform. This change reduces code duplication, improves maintainability, and centralizes QP boundary logic while preserving existing rate control behavior across all platforms.