codec2 (non-mainline): pin cpp_std to gnu++17

This project doesn't build with gnu++20 and prevents AOSP default to be
upgraded to a newer standard. Once the build failure is fixed here, this
change can be reverted.

Bug: 311052584
Test: m checkbuild
Change-Id: I4feafea879301de6d7fe1240ce3298d99da1b3d6