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This commit refactors ash/accelerometer that if group iioservice exists,
registering to Sensor Hal Dispatcher and waiting for samples updated
from iioservice instead of reading on sysfs files directly for
accelerometers' data.

BUG=b:172208566, b:172414227, b:168434557
TEST=builds, unit tests, and run on octopus & jacuzzi. Accels' samples
are updated successfully, and the auto-rotate screen is working.

Cq-Depend: chromium:2470222
Change-Id: Icf65705b2b8facb7fe79423edba163639e2ae45f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2345942Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Cheng-Hao Yang <chenghaoyang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827626}