weaved: Hook up long-poll HTTP requests for /privet/v3/checkForUpdates

libweave now supports log-poll HTTP requests for device state change
notifications as described in:
https://developers.google.com/cloud-devices/v1/reference/local-api/check_for_updates

Made corresponding implementation changes on weaved side.

BUG: 23908251
Change-Id: I60f9ad67907645b4164a5e8cf716a0a31e760d72
2 files changed