Added hysteresis check to screen brightness.

Previously, the AutoBrightnessController would suppress updates if
the ambient brightness hasn't changed enough, to avoid minor changes
that wouldn't really matter but probably annoy the user.
However, if the brightness curve was rather flat, a large change in
the ambient brightness might translate to a small and unnecessary
change in the screen brightness that would still get applied. So,
this CL adds similar hysteresis checks for the screen brightness.

Test: manual - simulate an ambient brightness that would cause a
      minor change in the screen brightness, and make sure it is
      suppressed (and the appropriate logs are emitted).
Change-Id: I41aefb5a9a82c9b598cf31f374f338899ffc9b11