Change Android mv() back to using the system renameTo() method.

A previous change to rn-fetch-blob's mv implementation entailed copying the source file to the destination file and then deleting the source file.  This approach has an undesirable impact on the mv operation, where the operation takes orders of magnitude longer to complete.  It also adds disk space requirements that shouldn't be necessary, since both files need to exist at the same time.
This change goes back to using the renameTo() method, while adding some extra error checking to hopefully alleviate whatever problems caused the original fix.
Note that the POSIX mv() and rename() methods both stipulate that moving to a non-existent directory should be treated as an error.

Author: pcardon

android/src/main/java/com/ReactNativeBlobUtil/ReactNativeBlobUtilFS.java

@@ -648,22 +648,23 @@ static void mv(String path, String dest, Callback callback) {
         }
 
         try {
-            InputStream in = new FileInputStream(path);
-            OutputStream out = new FileOutputStream(dest);
-
-            //read source path to byte buffer. Write from input to output stream
-            byte[] buffer = new byte[1024];
-            int read;
-            while ((read = in.read(buffer)) != -1) { //read is successful
-                out.write(buffer, 0, read);
+            // mv should fail if the destination directory does not exist.
+            File destFile = new File(dest);
+            File parentDir = destFile.getParentFile();
+            if (parentDir != null && !parentDir.exists()) {
+               callback.invoke("mv failed because the destination directory doesn't exist");
+               return;
+            }
+            // mv overwrites files, so delete any existing file.
+            if (destFile.exists()) {
+                destFile.delete();
+            }
+            // mv by renaming the file.
+            boolean result = src.renameTo(destFile);
+            if (!result) {
+                callback.invoke("mv failed for unknown reasons");
+                return;
             }
-            in.close();
-            out.flush();
-
-            src.delete(); //remove original file
-        } catch (FileNotFoundException exception) {
-            callback.invoke("Source file not found.");
-            return;
         } catch (Exception e) {
             callback.invoke(e.toString());
             return;