[build-tools] Add Android web previews

[szdziedzic]

Summary

Adds Android web previews to the existing EAS remote-session flow by selecting the preview server from the worker runtime platform.

• keep @expo/serve-sim for Darwin/iOS
• launch @expo/serve-emu@latest with WebRTC H.264 for Linux/Android
• share Cloudflare TURN retrieval, readiness polling, ngrok tunneling, and cleanup
• configure Android previews for 1280px, 3 Mbps, 30 fps, and one-second keyframes
• pass Appium's exact emulator serial to serve-emu
• report webPreviewUrl for Agent Device, Argent, and Appium sessions on both platforms
• make the EAS CLI preview label platform-neutral

Depends on expo/serve-emu#8 and expo/serve-emu#9. The scoped npm package must be bootstrapped before this integration is deployed.

No WWW or GraphQL schema change is needed: all three remote-config variants already store and render webPreviewUrl.

Verification

• 48 focused build-tools tests
• 3 focused eas-cli tests
• build-tools and eas-cli typechecks
• type-aware oxlint with zero warnings
• targeted oxfmt check
• exact serve-emu invocation validated against a headless Android AVD