Fix bug where notification text colors stop updating

ColorExtractor adds onColorsChangedListener as WeakReference
so we should not pass it in as anonymous lambda; otherwise it gets
garbage collected eventually and notif text colors stop updating.

This change adds persistent onColorsChangedListener to NSSLC.

Fixes: 173107499
Bug: 172289783
Test: visual - white text on dark background and vice versa
Change-Id: I42ee25fdf2ff9b40b78bae38416004249603444d