Public API for getting a persistent VD ID.

The VD deviceId is ephemeral and cannot be used for storing device
preferences by apps. Exposing the CDM association ID for the
corresponding physical device solves this (we're not exposing the
semantics of the ID, just calling it "persistent").

Bug: 286363466
Test: atest VirtualDeviceTest
Test: atest VirtualDeviceManagerBasicTest
Change-Id: Iacfcc7d725b9c6c0f364e113efe18ce0a63409a3