Improve auto-brightness hysteresis.

Reintroduced the stability time heuristic which requires brightness
to remain significantly above or below the currently accepted
ambient brightnes before effecting a brightness change.  The
heuristic has the nice property of preventing light sensor noise
from causing oscillations in brightness even when the noise has
a relatively large magnitude (such as in low light environments).

The time bound and filter thresholds are current set so that
brightness increases typically occur within 5 seconds of a change
in the ambient environment.  Decreases take somewhat longer and
typically occur within 10 seconds.

Changed the timing for brightness animations when the screen is
being dimmed due to a pending user activity timeout.  The screen
now dims slowly but then brightens rapidly when touched.
Previously the screen dimmed quickly and brightened slowly which
felt somewhat unresponsive.

Fixed a problem where a brightness change might not occur because
the light sensor had not reported a new value in a long time.
Now we synthesize measurements when needed to ensure that a
transition will take place if appropriate.

Bug: 7387800
Change-Id: I998df2fec59922042a41a1ba4af97ea52c0bd02a