dimdisplay: only dim the screen at configured dim time

The current logic is completely broken:

  1. It disables the screen at m_dimOnIdleTime even though disabling the screen is handled elsewhere with a separate timeout, and the logic here has prefix "Dim*" so has nothing to do with disabling it.
  2. It does not follow the timeout configured by a user (that is the m_dimOnIdleTime) and instead dims the screen at completely arbitrary 50% and 75% periods of the configured time.

Let's get rid of all that and only do exactly what the user asked: dim the screen at m_dimOnIdleTime.

BUG: 304696

Edited by Nate Graham

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