Migrate CA certificates to all users

Copies the /data/misc/keychain/cacert-* directories to all users on
the device, whereas previously they were simply copied to user 0.

This is a shallow copy so anything that wasn't supposed to be there
will disappear.

Bug: 17811821
Change-Id: Iae5909ab8d5efdb83c9c8fdf0e10ab7060d022cc