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Updating with CL:2386879, this commit adds
SensorServiceNewDevicesObserver to support sensors that are setup after
iioservice starts, and let iioservice notify sensor clients that new
sensors are available.
This commit also adds the implementation of the new method in the fake
class.

BUG=b:172414309
TEST=builds

Change-Id: Ib2bfba1038dc737336424699bfabf02b6ae68941
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2523131Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Commit-Queue: Cheng-Hao Yang <chenghaoyang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825268}