Refrain from overwriting shortcut icon with default icon

Invert the condition check in IconCache so that, if a shortcut is
already using a non-default icon, do not replace it with default icon
again.

Bug: 162783875
Test: add weather shortcut, backup, restore, verify its icon is not
replaced by default droid icon

Change-Id: Idaf4ce4afa84d21f7192a2f4ce41a6f8daa8d3f2
(cherry picked from commit 61e8ae1d)