Fix WallpaperCropper key error.

Fix an error introduced when trying to replace Map<Int, Rect> with
SparseArray<Rect>. It's clearly wrong to iterate over a SparseArray with
a "for" loop, then use the "i" index on another SparseArray.

Flag: ACONFIG com.android.window.flags.multi_crop DEVELOPMENT
Bug: 270726737
Test: there was a clear issue in some cases, manually debugged it

Change-Id: I3d78a8c958a05b5b2f55eb30d1d1bf4972c2170b