Files under the build_overrides directory in the buildroot are typically hardcoded imports in third-party dependencies used for project-specific configuration. For example, ANGLE hardcodes an import of //build_overrides/angle.gni so that consumers of ANGLE can configure the build to their needs.

We can add a copy all existing src/build_overrides files at the equivalent src/flutter/build_overrides path in the engine. A followup patch will replace each existing file under src/build_overrides in the buildroot with a shim that just imports the files landed in this patch. This allows the upstream project to continue hardcoding the //build_overrides/foo.gni path, but provides a seamless path forward
when we drop the buildroot.

Part of #67373.