[P3] CI: one system with PR checks (build, lint, tests) and Gradle cache

[amaury901130]

Pick Bitrise or GitHub Actions (remove the other). PR pipeline builds, lints, runs unit + screenshot tests, uses Gradle cache. Update README.

Acceptance criteria

• PR pipeline under 10 minutes
• Only one CI system documented

Reference: Android Base — Modernization Audit & Recommendations — section: Findings > Tooling, tests and CI