Expose syspem API for querying CDM associations

We want to restrict some BT capabilities(e.g. unpair device) to only the package's
associated device.
Since BT will become a mainline module, this needs to be exposed as a @SystemApi

Test: ateast android.os.cts.CompanionDeviceManagerTest#testGetAllAssociations
Bug: 165951651
Change-Id: Iab0da9352671e0b82ac855f91a6625c810946d17