a11y: Make images optional for single choice list view dialog

This makes `ItemInfoArrayAdapter.ItemInfo` support text-only choices. It
is to better accommodate the new dialogs.

Bug: b/389991440
Flag: com.android.settings.accessibility.enable_magnification_cursor_following_dialog
Test: None
Change-Id: If7514631a72631cf0fce85623fe30c8261f678c9