Document Android local build prerequisites and smoke test

Android/README.md

@@ -1 +1,6 @@
 # Google AI Edge Gallery (Android)
+
+The Android project lives in `Android/src`.
+
+For local build and run instructions, see the repository
+[`DEVELOPMENT.md`](../DEVELOPMENT.md#android-app).

DEVELOPMENT.md

@@ -1,11 +1,54 @@
 # Development notes
 
-## Build app locally
+## Android app
 
-To successfully build and run the application through Android Studio, you need to configure it with your own HuggingFace Developer Application ([official doc](https://huggingface.co/docs/hub/oauth#creating-an-oauth-app)). This is required for the model download functionality to work correctly.
+The source tree currently contains the Android app under `Android/src`.
+
+### Prerequisites
+
+- Android Studio with its bundled JDK, or JDK 17 or newer for command-line builds.
+- Android SDK Platform for the app's `compileSdk` version in
+  [`Android/src/app/build.gradle.kts`](Android/src/app/build.gradle.kts).
+- Android SDK Build-Tools installed through Android Studio's SDK Manager.
+- Network access for the first Gradle sync/build so Gradle can download the wrapper distribution,
+  Android Gradle Plugin, and project dependencies.
+
+Android Studio normally configures the JDK and SDK paths for you. For command-line builds,
+make sure `ANDROID_HOME` points to your Android SDK installation before running Gradle.
+
+### Credential-free build check
+
+From a fresh clone, verify that the project syncs and compiles before configuring any external
+services:
+
+```shell
+cd Android/src
+./gradlew :app:assembleDebug
+```
+
+This builds a debug APK with the checked-in placeholder Hugging Face OAuth values. It is useful as
+a local build check, but model download sign-in will not work until you complete the Hugging Face
+configuration below.
+
+### Hugging Face OAuth configuration
+
+To run the full application flow with model download support, configure your own Hugging Face
+Developer Application ([official doc](https://huggingface.co/docs/hub/oauth#creating-an-oauth-app)).
 
 After you've created a developer application:
 
-1. In [`ProjectConfig.kt`](https://github.com/google-ai-edge/gallery/blob/main/Android/src/app/src/main/java/com/google/ai/edge/gallery/common/ProjectConfig.kt), replace the placeholders for `clientId` and `redirectUri` with the values from your HuggingFace developer application.
+1. In [`ProjectConfig.kt`](Android/src/app/src/main/java/com/google/ai/edge/gallery/common/ProjectConfig.kt),
+   replace the placeholders for `clientId` and `redirectUri` with the values from your Hugging Face
+   developer application.
+
+1. In [`app/build.gradle.kts`](Android/src/app/build.gradle.kts), modify the
+   `manifestPlaceholders["appAuthRedirectScheme"]` value to match the redirect URL you configured
+   in your Hugging Face developer application.
+
+### Run the app
+
+Open `Android/src` in Android Studio, select an Android 12 or newer device or emulator, and run the
+`app` configuration.
 
-1. In [`app/build.gradle.kts`](https://github.com/google-ai-edge/gallery/blob/c1b50e160a66d5ea2ec2d8d8e63088b3cc0761bc/Android/src/app/build.gradle.kts#L41-L44), modify the `manifestPlaceholders["appAuthRedirectScheme"]` value to match the redirect URL you configured in your HuggingFace developer application.
+External boundary: a Hugging Face account and OAuth application are required for model download
+sign-in. Device/emulator availability and model downloads are not covered by the Gradle build check.