Document Android local build prerequisites and smoke test

[archit-goyal]

Summary

• Clarifies that the Android app source lives under Android/src.
• Documents Android Studio/JDK/SDK/network prerequisites for local development.
• Adds a credential-free ./gradlew :app:assembleDebug build check before Hugging Face OAuth setup.
• Separates basic compile verification from the full model-download runtime flow.
• Points Android/README.md to the development instructions.

Why

I tried a fresh-clone setup pass and found the Android build docs assumed local Android/JDK setup and Hugging Face OAuth context before giving contributors a basic build check. This makes the setup boundary explicit and gives contributors a smaller first command to verify the project syncs and compiles.

Verification

• Ran git diff --check.
• Attempted ./gradlew :app:assembleDebug from a fresh clone; local verification is blocked on this machine because Java/Android Studio/Android SDK are not installed.

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[archit-goyal]

Hi maintainers, just gently following up on this PR. Please let me know if there is anything I should adjust or any extra verification that would help. Thanks!