Update system palette APIs

UX landed on a format where we have 2 neutral palettes, and 3 accent
palettes. It's the ideal format to play with elevation and hue rotation,
in order to have a more vibrant and less muddy UI.

Fixes: 181986389
Bug: 173553055
Test: atest SystemPalette
Test: atest ThemeOverlayControllerTest
Test: atest ThemeOverlayApplierTest
Test: atest DeviceDefaultThemeTest
Change-Id: I80d3f7d1cc92e97efcb40fe6dc9f09918321d273