contexthub: add findEndpoints(String) for service discovery

- Add findEndpoints(String) to allow client to query for the endpoints
  with service descriptor.

Test: build
API-Coverage-Bug: 377554469
Bug: 375487784
Bug: 379323274
Flag: android.chre.flags.offload_api
Change-Id: I7016caa851b96a521bb86656d42795c3b1e9d405