contexthub: add 10s timeout for open request

- Use a shared ScheduledThreadPoolExecutor in ContextHubEndpointManager
  for all brokers.
- Remove session when timeout, and close the session with Timeout as
  reason.
- When session open request is pending, ignore duplicated open session
  request.

Test: atest ContextHubEndpointTest
Fix: 387716930
Flag: EXEMPT, bug fix
Change-Id: I3e6ba00f87d2ddd40969a81802503abb2e9a5af6