Require protobuf for build flags

This change forces build flags from .scl to .textproto format.

This change resubmits commit 8663122e
such that it will merge downstream.

Bug: 328495189
Bug: 338450506
Test: manual, TH
Ignore-AOSP-First: DNMA in aosp-main to avoid aosp-main-future
Change-Id: I024f620b55e5e655f2b7b4764b217744334f7995