add android support

android/README.md

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+# Oral Trainer Android SDK
+
+Android SDK for online oral-training video playback. It provides an embeddable
+Media3/ExoPlayer player, streaming cache, tablet gestures, sentence navigation,
+and a placeholder interface for future imitation-quality scoring.
+
+## Modules
+
+- `oral-trainer-sdk`: Android library module to publish as an AAR.
+- `sample-app`: Minimal Android app showing SDK integration.
+
+## Default Gesture Mapping
+
+- Single tap on the playback area: play / pause.
+- Swipe left: previous sentence; if there is no sentence data, rewind by the
+  configured seek step.
+- Swipe right: next sentence; if there is no sentence data, fast-forward by the
+  configured seek step.
+
+These defaults match the desktop player's arrow-key workflow while fitting a
+tablet touch screen.
+
+## Basic Integration
+
+```kotlin
+val sdk = OralTrainerSdk.init(context)
+val controller = sdk.createController(
+    playerConfig = PlayerConfig(
+        sentenceMode = true,
+        defaultSeekStepMs = 10_000L,
+        autoPlay = false,
+    )
+)
+
+val playerView = OralTrainerPlayerView(context)
+playerView.bind(controller)
+
+controller.loadItem(
+    TrainingMediaItem(
+        id = "lesson_01",
+        title = "Lesson 01",
+        uri = Uri.parse("https://cdn.example.com/lesson_01.mp4"),
+        customCacheKey = "lesson_01",
+        sentences = listOf(
+            SentenceBoundary(0, 0L, 4200L, "Listen and repeat."),
+            SentenceBoundary(1, 4200L, 9000L, "Swipe to jump by sentence."),
+        ),
+    )
+)
+```
+
+## Online Streaming And Cache
+
+The SDK uses AndroidX Media3 `SimpleCache` through `CacheDataSource.Factory`.
+It supports regular MP4 streams plus HLS and DASH through Media3. Configure the
+cache location and maximum size at initialization:
+
+```kotlin
+OralTrainerSdk.init(
+    context,
+    OralTrainerSdkConfig(maxCacheBytes = 1024L * 1024L * 1024L)
+)
+```
+
+## Future Imitation Scoring
+
+Provide an implementation of `ImitationQualityAssessor` when the speech
+assessment algorithm is ready:
+
+```kotlin
+val controller = sdk.createController(
+    imitationAssessor = MyImitationQualityAssessor()
+)
+```
+
+Then call `assessCurrentSentence(recordingUri, callback)` after the student
+records a sentence.
+
+## Build
+
+Prerequisites: JDK 17 or newer and Android SDK Platform 36.1. Open the
+`android/` directory in Android Studio, or run:
+
+```bash
+ANDROID_HOME="$HOME/Library/Android/sdk" ./gradlew :oral-trainer-sdk:assembleDebug
+```
+
+If the Gradle ZIP has already been downloaded, extract it and select the
+extracted directory in Android Studio under `Settings > Build, Execution,
+Deployment > Build Tools > Gradle > Gradle distribution > Local installation`.
+For example, the local installation directory on the development machine is
+`/Users/liushuming/Downloads/ssss/gradle-8.13`.