Refactor flutter to support Android (#1072)

flutter/sherpa_onnx_android/android/.gitignore

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+*.iml
+.gradle
+/local.properties
+/.idea/workspace.xml
+/.idea/libraries
+.DS_Store
+/build
+/captures
+.cxx

flutter/sherpa_onnx_android/android/build.gradle

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+// The Android Gradle Plugin builds the native code with the Android NDK.
+
+group = "com.k2fsa.sherpa.onnx.sherpa_onnx_android"
+version = "1.0"
+
+buildscript {
+    repositories {
+        google()
+        mavenCentral()
+    }
+
+    dependencies {
+        // The Android Gradle Plugin knows how to build native code with the NDK.
+        classpath("com.android.tools.build:gradle:7.3.0")
+    }
+}
+
+rootProject.allprojects {
+    repositories {
+        google()
+        mavenCentral()
+    }
+}
+
+apply plugin: "com.android.library"
+
+android {
+    namespace 'com.k2fsa.sherpa.onnx'
+
+    // Bumping the plugin compileSdk version requires all clients of this plugin
+    // to bump the version in their app.
+    compileSdk = 34
+
+    // Use the NDK version
+    // declared in /android/app/build.gradle file of the Flutter project.
+    // Replace it with a version number if this plugin requires a specific NDK version.
+    // (e.g. ndkVersion "23.1.7779620")
+    ndkVersion = android.ndkVersion
+
+    compileOptions {
+        sourceCompatibility = JavaVersion.VERSION_1_8
+        targetCompatibility = JavaVersion.VERSION_1_8
+    }
+
+    defaultConfig {
+        minSdk = 21
+    }
+}

flutter/sherpa_onnx_android/android/settings.gradle

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+rootProject.name = 'sherpa_onnx_android'

flutter/sherpa_onnx_android/android/src/main/AndroidManifest.xml

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+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+  package="com.k2fsa.sherpa.onnx">
+</manifest>

flutter/sherpa_onnx_android/android/src/main/jniLibs/README.md

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+# Introduction
+
+Pre-built libs are not checked-in.
+
+Please use
+
+ - https://github.com/k2-fsa/sherpa-onnx/blob/master/build-android-arm64-v8a.sh
+ - https://github.com/k2-fsa/sherpa-onnx/blob/master/build-android-armv7-eabi.sh
+ - https://github.com/k2-fsa/sherpa-onnx/blob/master/build-android-x86-64.sh
+ - https://github.com/k2-fsa/sherpa-onnx/blob/master/build-android-x86.sh
+
+The following is an example for `arm64-v8a`:
+
+```bash
+git clone https://github.com/k2-fsa/sherpa-onnx
+cd sherpa-onnx
+
+export SHERPA_ONNX_ENABLE_JNI=OFF
+export SHERPA_ONNX_ENABLE_C_API=ON
+./build-android-arm64-v8a.sh
+
+cp -v build-android-arm64-v8a/install/lib/*.so flutter/sherpa_onnx_android/android/src/main/jniLibs/arm64-v8a/
+```