Independent build-location migration

The existing Kokoro-Hexagon build directory was moved from the adjacent a local build directory location to this repository's ignored build/ directory. The source location is absent; the destination has the same 140 files and total byte count, and the existing APK's SHA-256 is unchanged. No new APK was built or installed for this migration.

setup-kokoro.ps1 now defaults its APK and optional intermediates to build/. The repository write guard permits only the approved APK and intermediates there; it still rejects repository-local signing keys and package-cache paths. The repository-change check excludes only build/, and Git ignores it. An existing package cache moved with the historical build folder; the script's default package-cache location remains outside the repository.

Checks: PowerShell source parsed; Step 1 -WhatIf showed the new default write plan and reported no source changes; paths outside build/ were rejected for the APK and intermediates; repository-local key and cache paths were rejected; the moved managed assembly passed its Windows load test; and the production-closure source gate passed. A full build at the new default path remains unverified.