Fix incorrect redirection when staging SSH files.

Existing code uses sudo and cat with redirection to stage SSH files
(e.g. id_rsa). But the redirection redirects the output of the sudo
command as well as the cat command. Thus, any stdout from the
interactive shell (e.g. .bashrc) can end up in the staged files.
Remove the redirection and use dd instead (as is used to copy
staged files to their final destination).

Closes-Bug: #1266557
Change-Id: Ia976bc309a282e4917e89afca52b6986bab26f07
Merge "Fix incorrect redirection when staging SSH files."