Several changes to VDM/CDM logic.

 - return a non-null persistent id for the default device
 - Add overloads to CDM hidden APIs to get association info for all
   users
 - Maintain a list of all associations in VDMS
 - Add a VDMInternal listener for when a association that may create
   virtual devices is gone, so the persistent id is gone as well

Fix: 295258915
Fix: 282629983
Test: atest VirtualDeviceManagerServiceTest
Test: atest VirtualDeviceManagerBasicTest
Change-Id: I0d33c7242652ca49aa68e8d912bcc263a7e4a08b