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: I7c229028d216c56a0fe4fb2a38ea65bed2e6906a
Merged-In: I41aefb5a9a82c9b598cf31f374f338899ffc9b11
Fixes: 122162842