Add Android demo for speaker recognition (#536)

See pre-built Android APKs at 
https://k2-fsa.github.io/sherpa/onnx/speaker-identification/apk.html

sherpa-onnx/csrc/speaker-embedding-extractor.h

@@ -9,6 +9,11 @@
 #include <string>
 #include <vector>
 
+#if __ANDROID_API__ >= 9
+#include "android/asset_manager.h"
+#include "android/asset_manager_jni.h"
+#endif
+
 #include "sherpa-onnx/csrc/online-stream.h"
 #include "sherpa-onnx/csrc/parse-options.h"
 
@@ -40,6 +45,11 @@ class SpeakerEmbeddingExtractor {
   explicit SpeakerEmbeddingExtractor(
       const SpeakerEmbeddingExtractorConfig &config);
 
+#if __ANDROID_API__ >= 9
+  SpeakerEmbeddingExtractor(AAssetManager *mgr,
+                            const SpeakerEmbeddingExtractorConfig &config);
+#endif
+
   ~SpeakerEmbeddingExtractor();
 
   // Return the dimension of the embedding