Add androidx suppport.

- Replace `android.support.annotation.Nullable` with 
`androidx.annotation.Nullable` import.
- Thanks to @maieonbrix hard work finding this fix. 
(itinance#686 (comment))

android/src/main/java/com/rnfs/RNFSManager.java

@@ -8,7 +8,7 @@
 import android.os.Environment;
 import android.os.StatFs;
 import android.provider.MediaStore;
-import android.support.annotation.Nullable;
+import androidx.annotation.Nullable;
 import android.util.Base64;
 import android.util.SparseArray;
 import android.media.MediaScannerConnection;