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As some clients, such as generic sensor, may connect to the same
SensorDevice multiple times, this commit refactors
chromeos::sensors::FakeSensorDevice to support that.

BUG=chromium:1006141
TEST=builds and unit tests of generic sensor

Change-Id: Ie792ab60daae8d9941d00fce6c7d3a248f9b8c57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2491767
Commit-Queue: Cheng-Hao Yang <chenghaoyang@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820675}