Make WallpaperPreferenceController available when only style picker present.

WallpaperPreferenceController prefers the style picker over the basic wallpaper picker. However, the entire setting is disabled when only the style picker is available, even though the basic wallpaper picker is not used. This change corrects that.

Fixes: 162395248
Test: Added unit tests. Also built, flashed and tested setting with all four combinations of CategoryPickerActivity and CustomizationPickerActivity enabled/disabled.
Change-Id: I4184f834ed771359cef13b64aacf2072e592d6d8