Add neutral palette, rename main and accent

Added new netrual colors palette, and renamed main and accent palettes
to primary and secondary respectively.
This was done to align with the new color spec.

Test: atest SystemPalette
Test: atest ColorTest
Test: atest ThemeOverlayControllerTest
Bug: 171916625
Change-Id: Ia56eb1e96a01c3ff8a64de0e1d632d4436bc9048