sensors: Move SensorReader::thread_checker_ to PollingSensorReader

This does not have any visible effects, but it improves design: whether to
use a ThreadChecker, a SequenceChecker or nothing at all is part of the
implementation strategy of each SensorReader subclass, so |thread_checker_|
shouldn't be in SensorReader itself.

Bug: 941559
Change-Id: Ifb9eb5a8460077ae7ae96d7ade79537257c63812
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1525937
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Auto-Submit: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641285}