[IOT-1520] Allow MOT to be disabled after it is enabled.

[IOT-1520] Allow MOT to be disabled after it is enabled.

These changes do the following:
1. Fix updateWriteableProperty to honor the 'mom'
   property any time that it is changed.
2. Rename updateWriteableProperty to
   DoxmUpdateWriteableProperty to clarify its usage.
3. Fix an invalid check in the ProvisioningClient.
4. Added tests to ensure that mom can be enabled
   and disabled when MOT is available.

Change-Id: I97c01a9dc8f44bcf5a6aeac3bf459777c97e19b0
Signed-off-by: Alex Kelley <alexke@microsoft.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/18109
Reviewed-by: Kevin Kane <kkane@microsoft.com>
Reviewed-by: Nathan Heldt-Sheller <nathan.heldt-sheller@intel.com>
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Pawel Winogrodzki <pawelwi@microsoft.com>
Reviewed-by: Greg Zaverucha <gregz@microsoft.com>