contexthub: create basic endpoint session message API

- Create a basic parcelable for HubMessage.
- Create interface definitions for bi-directional messaging in IContextHubEndpoint and IContextHubEndpointCallback.
- Connect callbacks for messages that requires ACK.

Test: manual
API-Coverage-Bug: 377554469
Bug: 375487784
Flag: android.chre.flags.offload_api
Change-Id: I231ee01335a93469cbdbc1d986ce017be75a6d23