APIs for developers to use DeviceDefault.DayNight.

Without this, developers would have to create their own DayNight version
for DeviceDefault. Adding the ThemeOverlay.DeviceDefault.Accent.DayNight
makes it easier to developers to retrieve accent color in xml.

Bug: 123999734
Test: make
Change-Id: I85f0ad6a467b7a6eb0626ce0bc9808b0559c3fa6