Fix crash on drag using mouse on large screen layout

When a mouse attempts to drag in list view whilst in the large screen
layout, it currently crashes with a null pointer reference. The field
`mIconLayout` was being referenced in the `inDragRegion` function,
however, the `layout-sw720dp/item_doc_list.xml` uses `@android:id/icon`
which doesn't create the ID correctly.

Bug: 356938552
Test: Attach mouse and attempt drag in all modes
Flag: EXEMPT bugfix
Change-Id: I6234bdc70d2f05086331608a4cc5e874ba2e7c23