Fix the UI of the preview pager broken when only wallpaper tab exists and the display size is largest. am: 8fbef256 am: eaa568ef

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/apps/ThemePicker/+/11970741

Change-Id: Ib5d08abf94ab055cdfbdbad523d1dc96ce2964db