Fixed inaccurate brightness slider UI

The brightness slider's sun icon now corresponds directly with where the
user taps. The user can tap both edges of the slider to easily get the
maximum and minimum brightness levels.

Fixes: 205967506
Test: Manually tested, tapped both edges of the slider and was able to easily get the max and min widths
Change-Id: I7611946edd39adb75bfbc7b990c7d9dafa105e9f