After shutting down AP, hibernate EC if battery is low

This preserves more battery for the real-time clock.  It also gets rid
of one battery level/state, since the EC will now shut down at the
same level (2%) as the AP.

Having the EC hibernate at the same level as the AP shuts down also
prevents the following weird scenario:

- EC powers on at 2% battery.
- EC won't power on the AP to do software sync because battery is low.
- But the charge state machine won't actually hibernate the EC until 1%.
- Between 2% and 1%, AC is inserted and system starts charging.
- But EC-RO is what's doing the charging; we should go through software sync
  before charging.

BUG=chrome-os-partner:17124
BRANCH=link
TEST=manual

On battery power,
1) With system on, battfake 2 -> system shuts down and EC hibernates
2) With system off, battfake 2 -> EC hibernates

Change-Id: I87d17f70d60673f3e3f1c6eb88f3f00a8c9ed4e7
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/44314

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