cmsdk: Add constraints and behaviors for our preferences

 * Add a new set of attributes which can be declared in the
   preference XML which will cause the preference to be
   removed from the hierarchy if the constraint is not met.
   Currently we can check for installed packages, system
   properties, config references, and system features.
 * CMPartsPreference is also updated to use similar logic
   based on the external XML.

Change-Id: If8fb39cd3312623e38bb3498bfb3f29ed8b444d6