Make colorContrast setting dependent on framework value

This CL aims to make sure that we directly use the ColorContrast
values emitted by framework instead of having a copy of it in the
WAllpaperPicker. We also use the getContrastValues() method provided by
frameworks to convert the contrast values.

Bug: 363461032
Test: Tested by building & installing picker on local and checking the
contrast values.
Flag: EXEMPT bugfix

Change-Id: I22806a0171a6dc3b0c09a4b69740f85e7bbf7721