Add #162 Android implementation

src/android/src/main/java/com/RNFetchBlob/RNFetchBlob.java

         RNFetchBlobReq.progressReport.put(taskId, config);
     }
 
+    @ReactMethod
+    public void df(final Callback callback) {
+        fsThreadPool.execute(new Runnable() {
+            @Override
+            public void run() {
+                RNFetchBlobFS.df(callback);
+            }
+        });
+    }
+
+
     @ReactMethod
     public void enableUploadProgressReport(String taskId, int interval, int count) {
         RNFetchBlobProgressConfig config = new RNFetchBlobProgressConfig(true, interval, count, RNFetchBlobProgressConfig.ReportType.Upload);

src/android/src/main/java/com/RNFetchBlob/RNFetchBlobFS.java

 import android.media.MediaScannerConnection;
 import android.net.Uri;
 import android.os.AsyncTask;
+import android.os.Build;
 import android.os.Environment;
+import android.os.StatFs;
 import android.os.SystemClock;
 import android.util.Base64;
 
         }
     }
 
+    static void df(Callback callback) {
+        StatFs stat = new StatFs(Environment.getDataDirectory().getPath());
+        WritableMap args = Arguments.createMap();
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR2) {
+            args.putString("internal_free", String.valueOf(stat.getFreeBytes()));
+            args.putString("internal_total", String.valueOf(stat.getTotalBytes()));
+            StatFs statEx = new StatFs(Environment.getExternalStorageDirectory().getPath());
+            args.putString("external_free", String.valueOf(statEx.getFreeBytes()));
+            args.putString("external_total", String.valueOf(statEx.getTotalBytes()));
+
+        }
+        callback.invoke(null ,args);
+    }
+
     /**
      * Remove files in session.
      * @param paths An array of file paths.