[power benchmarks] Support starting benchmark when power is plugged in.

With this CL, if a smart power strip is used, benchmark.py turns off
power before checking the environment, avoiding a failure.

Also, this CL refactors plug.py:
- No need to pass the device name for each method call.
- charge_to/discharge_to are symmetric.

Change-Id: Ie825afa8b4af379e1ae3bcb1c8bc3ec34bf352b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4080852
Reviewed-by: Olivier Li <olivierli@chromium.org>
Commit-Queue: Francois Pierre Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1082666}