SDR content brightness under HDR

[emoacht]

The functionality to adjust SDR content brightness under HDR is added in 4.12 for test purpose by a2d1479 and succeeding commits.

It can be enabed by Adjust SDR content brightness in the hidden menu. Please refer Probe about how to open the hidden menu.

There is still unsolved issue on how to reliably obtain the maximum value of SDR white level. As I pointed out in #411, the value of AdvancedColorInfo.MaxLuminanceInNits doesn't match actual maximum value of SDR white level.
So, if you find the slider value of this app doesn't match that of SDR content brightness in System Display, move the thumb of slider in System Display to the right end (maximum) so that this app can record the value at 100% and configure its slider.

You can check the raw values of AdvancedColorInfo in the probe.log at "Display Information" node.

[emoacht]

I have checked several HDR capable monitors whose maximum luminance range from 350 to 600 and found that the maximum value of SDR white level in nits is always 480. So I decided to fix the minimum at 80 nits and the maximum at 480 nits. It was done by 04769b1