PowerManagerService: Avoid paralysis when light events are reported frequently.

Avoid resetting the debounce timer for automatic brightness if a new light event
is received that agrees with the direction of change of the previous event(s).

Change-Id: Id4d71f6db46ded46b24eb44cb8de9b2cfedb3f06
Signed-off-by: Mike Lockwood <lockwood@google.com>