fix(brightness): adjust brightness slider bar layout width to fit in the window safe area

In BrightnessDialog it uses the window full width to calculate the
slider bar layout width. However the window full width may contain the
display screen cutout area. Therefore, we change to calculate the window
available width (for safe area) for the following slider bar width
calculation.

Bug: 347826058
Flag: EXEMPT bugfix
Test: manually
Change-Id: I2c6e94116759021356db82064c1a898a144a6d83