contexthub: add HubEndpointSession + IHubEndpointLifecycleCallback

- Introduce HubEndpoint and HubEndpointSession class
- Introduce IHubEndpointLifecycleCallback interface

- Add IContextHubEndpoint & IContextHubEndpointCallback that connects
  the lifecycle events to the client API
- Create implementation stub for ContextHubService

Test: build
API-Coverage-Bug: 377554469
Bug: 375487784
Flag: android.chre.flags.offload_api
Change-Id: I7b279ae0857c766c3d801197bb5c90b6cf9a3afe