DisplayColor: Properly handle a minValue

* Previously setting anything as a minValue would cause the
  percentages to get out of whack. For instance, setting it to 35
  would cause the maximum percentage to climb above 100% as well
  as still allowing the value to be set lower than the min.
* This makes the maximum for the slider the max value minus the min
  (so we get the proper spread for min to max) and then adds the min
  back on before setting so we can't possibly go below the min.

Change-Id: I8dcdc422f54847940351fae10c74af6301d1e1be