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This commit adds OnECLidAngleDriverStatusChanged in
AccelerometerReader::Observer, which simplifies implementations of
AccelerometerProviderInterface by removing the one-time-read.
This commit also implements AddObserver, RemoveObserver,
StartListenToTabletModeController, and StopListenToTabletModeController
in AccelerometerProviderInterface instead of the derived classes.

BUG=b:171446270, b:172414227
TEST=unit tests and run on kohaku(with iioservice) and helios(without
iioservice)

Change-Id: Ic0d37edc8df22cbd6facf54fe5fb0d11425be2b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2538855
Commit-Queue: Cheng-Hao Yang <chenghaoyang@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841313}