shill: cellular: Optimize events raised by MobileOperatorInfo.

MobileOperatorInfo posts events whenever network operator information changes.
Since these events are posted on the event loop, it is possible that multiple
events get posted before any of the earlier events are processed. In that
particular case, drop subsequent events, because there is no new information to
be processed.

BUG=chromium:370198
TEST=- Run shill_unittests.
     - Manually test (from logs) that network operator is identified correctly,
       and that multiple events are not raised for the same information.
     - Run cellular_ServiceName.
Change-Id: If46444c085681a59d7d6aa91a36bbcce1bcb183c
Reviewed-on: https://chromium-review.googlesource.com/199400
Reviewed-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>