Record app<->device association to xml file

Bug: 30932767
Test: Ensure file not exists -> query associations -> ensure result is empty list
  Associate device -> cat xml file -> ensure record appears as extected
  Disassociate device -> cat xml file -> ensure record is no longer present
Change-Id: Ibe456a6d9292e05e2391f5138e43fdaa37f87e1b