Add support for Android <= 7.1.1

This pull request for the issue related to objectbox#187 
Failed to load dynamic library (dlopen failed: library "libobjectbox-jni.so" not found)

Author: bm2ilabs

flutter_libs/android/src/main/java/io/objectbox/objectbox_flutter_libs/ObjectboxFlutterLibsPlugin.java

@@ -10,29 +10,42 @@
 
 /** ObjectboxFlutterLibsPlugin */
 public class ObjectboxFlutterLibsPlugin implements FlutterPlugin, MethodCallHandler {
-  /// The MethodChannel that will the communication between Flutter and native Android
-  ///
-  /// This local reference serves to register the plugin with the Flutter Engine and unregister it
-  /// when the Flutter Engine is detached from the Activity
-  private MethodChannel channel;
-
-  @Override
-  public void onAttachedToEngine(@NonNull FlutterPluginBinding flutterPluginBinding) {
-    channel = new MethodChannel(flutterPluginBinding.getBinaryMessenger(), "objectbox_flutter_libs");
-    channel.setMethodCallHandler(this);
-  }
-
-  @Override
-  public void onMethodCall(@NonNull MethodCall call, @NonNull Result result) {
-    if (call.method.equals("getPlatformVersion")) {
-      result.success("Android " + android.os.Build.VERSION.RELEASE);
-    } else {
-      result.notImplemented();
+    /// The MethodChannel that will the communication between Flutter and native Android
+    ///
+    /// This local reference serves to register the plugin with the Flutter Engine and unregister it
+    /// when the Flutter Engine is detached from the Activity
+    private MethodChannel channel;
+
+    @Override
+    public void onAttachedToEngine(@NonNull FlutterPluginBinding flutterPluginBinding) {
+        channel = new MethodChannel(flutterPluginBinding.getBinaryMessenger(), "objectbox_flutter_libs");
+        channel.setMethodCallHandler(this);
+        loadLibrary();
+    }
+
+    @Override
+    public void onMethodCall(@NonNull MethodCall call, @NonNull Result result) {
+        if (call.method.equals("getPlatformVersion")) {
+            result.success("Android " + android.os.Build.VERSION.RELEASE);
+        } else {
+            result.notImplemented();
+        }
+    }
+
+    @Override
+    public void onDetachedFromEngine(@NonNull FlutterPluginBinding binding) {
+        channel.setMethodCallHandler(null);
     }
-  }
 
-  @Override
-  public void onDetachedFromEngine(@NonNull FlutterPluginBinding binding) {
-    channel.setMethodCallHandler(null);
-  }
+    private static boolean isLibraryLoaded = false;
+
+    private static void loadLibrary() {
+        if (isLibraryLoaded) {
+            return;
+        }
+
+        // Loads `libobjectbox-jni.so`.
+        System.loadLibrary("objectbox-jni");
+        isLibraryLoaded = true;
+    }
 }