Add support for SDCards applications's directory (Android only) (#372)

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

         state = Environment.getExternalStorageState();
         if (state.equals(Environment.MEDIA_MOUNTED)) {
             res.put("SDCardDir", Environment.getExternalStorageDirectory().getAbsolutePath());
+            res.put("SDCardApplicationDir", ctx.getExternalFilesDir(null).getParentFile().getAbsolutePath());
         }
         res.put("MainBundleDir", ctx.getApplicationInfo().dataDir);
         return res;

fs.js

     DownloadDir : RNFetchBlob.DownloadDir,
     DCIMDir : RNFetchBlob.DCIMDir,
     SDCardDir : RNFetchBlob.SDCardDir,
-    MainBundleDir : RNFetchBlob.MainBundleDir
+    SDCardApplicationDir : RNFetchBlob.SDCardApplicationDir,
+    MainBundleDir : RNFetchBlob.MainBundleDir,
+    LibraryDir : RNFetchBlob.LibraryDir
 }
 
 /**