[IOT-1525] Keep credential resource encrypted on-disk on Windows

[IOT-1525] Keep credential resource encrypted on-disk on Windows

The Windows platform provides a way to encrypt and decrypt data
with system-managed keys. Since the cred resource contains
sensitive symmetric and private keys, keep this resource encrypted
on disk.

Change-Id: I88f3cab76f2e782ce778bc64830491822b7d9842
Signed-off-by: Kevin Kane <kkane@microsoft.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/15033
Reviewed-by: Alex Kelley <alexke@microsoft.com>
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Mike Fenelon <mike.fenelon@microsoft.com>
Reviewed-by: Randeep Singh <randeep.s@samsung.com>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>
Reviewed-by: Greg Zaverucha <gregz@microsoft.com>