The problem was that an automatically triggered 'idle' completion result
could come in while we were waiting for actual 'triggered' completion
and then the real completion result would be discarded.

To avoid that:
* Only accept proposals from the currently running completion.
* Increase the idle completion timeout to 400ms so it doesn't get
  triggered during regular typing.

Change-Id: Ibb2fa8907a2b67a94e092dfdc5a48fb50be06b65
Reviewed-by: Leandro Melo <leandro.melo@nokia.com>