Do not try starting inexistent services in init.rc.

The vold, netd, app_process and keystore binaries are not shipped
in Ubuntu Touch so do not try starting them as services in init.rc.
This prevents possibly confusing error messages like
"init: cannot find '/system/bin/vold', disabling 'vold'"
in the kernel logs.

Signed-off-by: Jani Monoses <jani@ubuntu.com>
Signed-off-by: Sergio Schvezov <sergio.schvezov@canonical.com>