Refactor flutter to support Android (#1072)

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

@@ -0,0 +1,3 @@
+<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/
+```