UnregisteredLocalFile.php: Override File::getBitDepth() stub
The File::getBitDepth() stub simply returns 0 and UnregisteredLocalFile was not
overriding this to return the actual bit depth. UnregisteredLocalFile::getBitDepth()
now handles this by returning the 'bits' field of $this->getImageSize().
This commit just fixes the above bug, but some rethinking of the structure and
behaviour of this class and the UploadStashFile class is required.
Bug: T36952
Change-Id: I4a8fd6b74b923f2052ccd2e47980486299ad2a6e