Remove hard-coded colors in Wallpaper Picker

The CLs in this topic remove the hard-coded colors from the app, replacing them with ?android:attr/, ?attr/, and sometimes @color/ that is defined using device default colors.

Fixes: 173549815
Test: none added
Change-Id: I4f0d18c1c4afdc5e1651e5b34fef8c7d71cdf8b7