[IOT-1449] mbedTLS: Use wcslen instead of lstrlenW on Windows

[IOT-1449] mbedTLS: Use wcslen instead of lstrlenW on Windows

lstrlenW is not allowed in Windows Store applications. Use wcslen
instead.

Also fix a nearby narrowing size_t to int conversion to make sure
it's safe.

Change-Id: I14d37b6a0d6c82444f4733f5d54bd8173ca62b71
Signed-off-by: Kevin Kane <kkane@microsoft.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/13183
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Dave Thaler <dthaler@microsoft.com>
Reviewed-by: Randeep Singh <randeep.s@samsung.com>