[Expressive design] Hide background of IllustrationPreference

This new requirement from UX and Animation team, we don't need to
show background the lottie and drawable should handle by them self

Fixes: 421797172
Fixes: 385768669
Test: visual
Flag: com.android.settingslib.widget.theme.flags.is_expressive_design_enabled
Change-Id: I235050cea0978015efca34a881789e7c1ebaf3c0