Add theme style flag to WallpaperColors

Theme colors should always be the same in sysui and launcher.
We're now sharing a hint HINT_SUPPORTS_DARK_THEME, to make sure
we're never out of sync.

Test: runtest -x tests/Internal/src/android/app/WallpaperColorsTest.java
Fixes: 63140091
Change-Id: Ibd196f540d77269df377804b0f4d4d0d20820067