In-app gradient color based on systemui theme.

When viewing your scrim on top of an app (wallpaper not visible),
the gradient color should be dark if your wallpaper is dark and
light if your wallpaper is light.

Change-Id: I5f3aea5bf9ec066b7caecd7cadfd2814e3758bd1
Fixes: 63121129
Bug: 63365056
Test: runtest -x tests/Internal/src/com/android/internal/colorextraction/types/TonalTest.java
Test: runtest -x packages/SystemUI/tests/src/com/android/systemui/colorextraction/SysuiColorExtractorTests.java
(cherry picked from commit c77b71d7)