WORKAROUND: mkbp: Remove KEY_BATTERY from valid keys.

WORKAROUND: mkbp: Remove KEY_BATTERY from valid keys.

  While previous ghosting fix:
    688c20f CHROMIUM: Strengthen ghosting algorithm.

  Did help rule out null keys it was thought it would also correctly
  identify 3 vs. 4 keypresses (sharing row/col) regardless of whether
  key was virtual (KEY_BATTERY) or real.  It does not however.

  This change migrates KEY_BATTERY to invalid key while algorithm is
  further refined.

BUG=chrome-os-partner:18814
TEST=manual
boot Spring plugged after an EC reset and see no ghosting issues for
sequences ctrl + u|w|n|t

Change-Id: I1038a18c353e84a52ad77e44e497c747d310addf
Signed-off-by: Todd Broch <tbroch@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/48674
Reviewed-by: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>