DisplayManager: Switch RampAnimator to ramp in HLG space.

This switches the ramp animator logic to ramp in the perceptual HLG
space, thereby making ramping appear linear to the human eye.

Bug: 204252940
Test: Verified on Pixel 6 Pro that ramping is much more linear and
      consistent.
Change-Id: I946e1570e654eb25aa334be827ca3d15cba21c48