Add theme mode design let user can choose theme self

Add a ContentObserver on WallpaperManagerService to
listen THEME_MODE value change. When changed, call
notifyWallpaperColorsChanged and set WallpaperColors
color hint by the current mode.

Bug: 63903361
Test: manual
Change-Id: I4f7aa7b184565b1bb23c7f4f1f07fc310dac7546