Refactors reason to common HubEndpoint.Reason

Also adds the reason parameter to the onEndpointsStopped discovery
callback.

API-Coverage-Bug: 377554469
Bug: 375487784
Flag: android.chre.flags.offload_api
Test: Compile

Change-Id: I4a0749bd56cdbce6a5eed739faa61c2b3b6cbeb2
6 files changed