Refactor flutter to support Android (#1072)

flutter/sherpa_onnx/lib/src/wave_writer.dart

@@ -0,0 +1,27 @@
+// Copyright (c)  2024  Xiaomi Corporation
+import 'dart:ffi';
+import 'dart:typed_data';
+import 'package:ffi/ffi.dart';
+
+import './sherpa_onnx_bindings.dart';
+
+bool writeWave(
+    {required String filename,
+    required Float32List samples,
+    required int sampleRate}) {
+  final Pointer<Utf8> filenamePtr = filename.toNativeUtf8();
+
+  final n = samples.length;
+  final Pointer<Float> p = calloc<Float>(n);
+
+  final pList = p.asTypedList(n);
+  pList.setAll(0, samples);
+
+  int ok =
+      SherpaOnnxBindings.writeWave?.call(p, n, sampleRate, filenamePtr) ?? 0;
+
+  calloc.free(p);
+  calloc.free(filenamePtr);
+
+  return ok == 1;
+}