Return a copy of the "real" set in AssociationStore

Return a copy of the "real" set AssociationSet from
AssociationStoreImpl.getAssociations(), which fixes
ConcurrentModificationException-s that arise if a direct reference to
the mIdMap.values() collection is returned.

Bug: 220018043
Test: atest CtsCompanionDeviceManagerCoreTestCases
Test: atest CtsCompanionDeviceManagerUiAutomationTestCases
Test: manually install companion app, create a CDM Association with a
      BLE companion device, uninstall the app.
Change-Id: Ie0ecab7fe61923ecef1541cb9f6450742c4b83b2