CTS SystemPalette: extend delta hue tolerance to 15

Test is failing on bramble because delta hue is 14, and required delta
hue is <=12. Change delta hue required to <= 15. 

Bug: 207209953
Test: atest -s localhost SystemPaletteTest
Change-Id: Ie49095960f68fefa452239fbc663a5553a565f46
1 file changed