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Add the SignalStrength routine to the ServiceConnection.

BUG=chromium:956783
TEST=1) chromeos_unittests --gtest_filter=CrosHealthdServiceConnectionTest.*
2) Applied SignalStrength changes and successfully ran the
SignalStrength routine on a DUT (verified using cros-health-tool diag
--action=run_routine --routine=signal_strength).

Change-Id: Iffa5fe58ee8c03e0fed30cfc984f7bf2d02a4f00
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343667Reviewed-by: Paul Moy <pmoy@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Oleh Lamzin <lamzin@google.com>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Kartik Hegde <khegde@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810899}