Introduce CDM AssociationStore

Add public (within system_server) AssociationStore interface that
provides APIs for retreiving and monitoring changes to the CDM
associations.
Add AssociationStoreImpl - implementation of the AssociationStore which
additionally adds adds methods for adding, removing and updating
associations to be used by CdmService.

Bug: 211398735
Test: atest CtsCompanionDeviceManagerCoreTestCases
Test: atest CtsCompanionDeviceManagerUiAutomationTestCases
Change-Id: I82bc948c9949c3034b539530ef9641baf220601c