Now that it doesn't need to leverage SWITCH_TASK to send the AC_CHANGE
notification, pure GPIO-based external power detection can move from
switch.c to its own file.
BUG=chrome-os-partner:18256
BRANCH=none
TEST=add AC power, UI shows charging indicator; remove AC, indicator goes away
Change-Id: Id495f34185b7d971c241ac6d0a8311a6bf544507
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/45789