[Android] Add a sample instrumentedTest (pytorch#800)

* [Android] Add a sample instrumentedTest

We can't add a pure Java unit test because we have Android JNI deps. We
use instrumented test instead. This is runnable if we have an emulator
on host.

Test: pushd android/Torchchat/; ./gradlew connectedAndroidTest; popd

* Add docs for LlamaModuleTest.java

android/Torchchat/app/src/androidTest/java/org/pytorch/torchchat/LlamaModuleTest.java

@@ -0,0 +1,49 @@
+package org.pytorch.torchchat;
+
+import androidx.test.ext.junit.runners.AndroidJUnit4;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.pytorch.executorch.LlamaCallback;
+import org.pytorch.executorch.LlamaModule;
+
+import static org.junit.Assert.*;
+
+/**
+ * Instrumented test, which will execute on an Android device.
+ *
+ * @see <a href="http://d.android.com/tools/testing">Testing documentation</a>
+ */
+@RunWith(AndroidJUnit4.class)
+public class LlamaModuleTest {
+    @Test
+    public void LlamaModule() {
+        LlamaModule module = new LlamaModule("/data/local/tmp/llm/model.pte", "/data/local/tmp/llm/tokenizer.bin", 0.8f);
+        assertEquals(module.load(), 0);
+        MyLlamaCallback callback = new MyLlamaCallback();
+        // Note: module.generate() is synchronous. Callback happens within the same thread as
+        // generate() so when generate() returns, all callbacks are invoked.
+        assertEquals(module.generate("Hey", callback), 0);
+        assertNotEquals("", callback.result);
+    }
+}
+
+/**
+ * LlamaCallback for testing.
+ *
+ * Note: onResult() and onStats() are invoked within the same thread as LlamaModule.generate()
+ *
+ * @see <a href="https://github.com/pytorch/executorch/blob/main/extension/android/src/main/java/org/pytorch/executorch/LlamaCallback.java">LlamaCallback interface guide</a>
+ */
+class MyLlamaCallback implements LlamaCallback {
+    String result = "";
+    @Override
+    public void onResult(String s) {
+        result += s;
+    }
+
+    @Override
+    public void onStats(float v) {
+
+    }
+}