When scanning a single file, take .nomedia into account

Make scanSingleFile behave the same way as scanMtpFile, by taking into
account whether there's a .nomedia file guarding the file being scanned.
Without this, downloaded (or otherwise explicitly scanned) images/video/music
will appear in Gallery and Music even if a .nomedia file is hiding them.

Change-Id: Ib9ad4bda1b9a942f79a37ccd8e6a54d57710f528