Use the canonical tmpdir for the default share_file

Rather than relaying on the environment or some hardcoded defaults, using File::Spec->tmpdir will *always* give you a working temp dir; this is important for systems that either don't have a standard tempdir (VMS) or simply don't have a tempdir (android, blackberry 10).

..truth be told though, this patch was written just for Android. But on the flip side, share_file will now actually use the correct tempdir on Windows.