Fix bug with resize frame disappearing.

Currently, if you resize the widget to the top, bottom or right, the
resize frame disappears. This is because the AppWidgetResizeFrame code
relies on the indices of the child elements to find the drag handles.

I updated the code to instead use IDs since that is more robust.

Test: manual
Bug: 182305670
Change-Id: Iaa9d32c9190fb579101914f03d57bb48dcecb88d