Improve the screen dim duration calculation.

Take into account whether the screen off timeout is very short.
If so, we use a shorter dim timeout.  Don't allow the dim
time to be more than 20% of the total screen on time so that
the screen remains bright at least 80% of the time even when
the timeout is short.

Bug: 7273646
Change-Id: Iccea764b90f0d8b1df7009d26160c6bcf6eabe5b