Add POC Android JNI integration

I want to stop using jgit and instead of libgit2 instead. I feel far
more confident with the latter, and I need to use it for ios anyway.

Currently I cannot use the latest version of jgit as that requires a
higher minSDKVersion, and I want to target older android devices as
well.

android/app/build.gradle

@@ -44,6 +44,11 @@ android {
         versionCode flutterVersionCode.toInteger()
         versionName flutterVersionName
         testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
+        externalNativeBuild {
+            cmake {
+                cppFlags ""
+            }
+        }
     }
 
     signingConfigs {
@@ -59,6 +64,11 @@ android {
             signingConfig signingConfigs.release
         }
     }
+    externalNativeBuild {
+        cmake {
+            path "src/main/cpp/CMakeLists.txt"
+        }
+    }
 }
 
 flutter {

android/app/src/main/cpp/CMakeLists.txt

@@ -0,0 +1,6 @@
+cmake_minimum_required(VERSION 3.4.1)
+
+
+add_library( native-lib
+             SHARED
+             native-lib.cpp )

android/app/src/main/cpp/native-lib.cpp

@@ -0,0 +1,10 @@
+#include <jni.h>
+#include <string>
+
+extern "C" JNIEXPORT jstring JNICALL
+Java_io_gitjournal_gitjournal_MainActivity_stringFromJNI(
+        JNIEnv* env,
+        jobject /* this */) {
+    std::string hello = "Hello from C++";
+    return env->NewStringUTF(hello.c_str());
+}

android/app/src/main/java/io/gitjournal/gitjournal/MainActivity.java

@@ -21,11 +21,24 @@ import io.flutter.util.PathUtils;
 // For MethodChannel
 
 public class MainActivity extends FlutterActivity implements MethodCallHandler {
+    static {
+        System.loadLibrary("native-lib");
+    }
+
+    /**
+     * A native method that is implemented by the 'native-lib' native library,
+     * which is packaged with this application.
+     */
+    public native String stringFromJNI();
+
+
     private static final String CHANNEL_NAME = "gitjournal.io/git";
     static MethodChannel channel;
 
     @Override
     protected void onCreate(Bundle savedInstanceState) {
+        Log.d("VISH", stringFromJNI());
+
         super.onCreate(savedInstanceState);
         GeneratedPluginRegistrant.registerWith(this);