PowerManagerService: Fix keyboard backlight support:

Turn off keyboard backlight immediately when keyboard is closed.
Changing keyboard visibility state signals user activity except in the case
where the keyboard is closed and the screen is already off.

Fixes bugs b/1351141 and b/1319625

Change-Id: If4bf2c1dd8e0c2bc08196e443b6103a6041f27e1
Signed-off-by: Mike Lockwood <lockwood@android.com>