contexthub: add HubEndpointSessionResult to allow accept/reject open session request

- HubEndpointSessionResult.accept() and reject(String) for results
- Call service.closeSession if rejected

Test: manual
API-Coverage-Bug: 377554469
Bug: 375487784
Flag: android.chre.flags.offload_api
Change-Id: I24065c9111cf3c321dec5f138e1487781c7ab8f0