fw: Repurpose PREVENT_POWER_KEY for preventing multiple system keys.

  Allows applications like camera, and various testing applications,
  to intercept certain system keys to verify behavior or leverage it
  in creative ways.

  Limited to signature/privileged.

Change-Id: Iacd8735af2aabafbf1fc062d8c565b7102dc6865
TICKET: FEIJ-522
(cherry picked from commit 936f7e26)