Make aodIconTint default to white.

Previously, this defaulted to NO_COLOR (0) when the wallpaperTextColor
cannot be fetched from the context, which made some icons not show up at
all.

Bug: 260377107
Test: manual testing (issue fully reproducible with the debug
notification icon) &  atest SystemUITests

Change-Id: Ia407251621fe11ec5daac76677d6b66bf8015fa2