mediatranscoding: 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: I3ce6d0c7244356c87244de7e44af60c34e892178