Tonal palette gradient types

The tonal palette now has 3 gradient types, one for regular scrims
another one for places that require stronger contrast, and a last
version for extra contrast.

Bug: 62161354
Test: runtest -x frameworks/base/packages/SystemUI/colorextraction/tests/src/com/google/android/colorextraction/ColorExtractorTest.java
Change-Id: I0be6726334f7a71f04ee02c61994101c97771f1a