Simplify BrightnessSynchronizer

Using the 0.0-1.0 scale for these operations is equivalent to using a constrained scale.
Therefore using the hardcoded PowerManager values is more efficient.
The context parameter has also been removed from these methods since this is no longer necessary to get stored PowerManager values.

Bug: 162398030

Test: manual - test with different brightness constraints.

Change-Id: Ie18e73b6f1b3effdbaf7e35eab14d51034442d07