charger: make default to not suspend in charge mode

  * http://r.cyanogenmod.com/#/c/19760 made the device suspend in
    charge mode when the animation was not displayed. Previously
    the device never suspended.
  * The above change broke charging mode on some (maybe many?) devices
    that do not properly handle suspending in charge mode.
  * This change brings back the more compatible default behavior of
    not suspending in charge mode and introduces a new BoardConfig.mk
    variable called BOARD_ALLOW_SUSPEND_IN_CHARGER that, when set to
    true, will suspend in charge mode.
  * This allows devices that support suspend in charger to opt-in
    instead of having to opt-out previously working devices.

Change-Id: I5a480e34ba5946a52e371ac79a965ecee5471c5a