Small safety checks in WallpaperCropper

If WallpaperCropper couldn't generate the crop, always remove any crop
guidelines. Also, always check in getCrop that all suggested crops are
valid. Also, avoid a potential infinite recursion in getAdjustedCrop.

Flag: ACONFIG com.android.window.flags.multi_crop DEVELOPMENT
Bug: NA, let's not spam bugs
Test: manual debug with logs
Change-Id: I3684d489f3dd7d5656db559b6e5cee5537e3bc5a