Don't crash in SecondaryUserController.

If icons were fetched before displayPreference() is called, it can
cause a NPE. This adds a null check to ensure that it never
attempts to set an icon without both the icon and preference being
non-null.

Change-Id: Ib4269428e64545c9e255f7e3292fb6c046c1f307
Fixes: 37248158
Test: Settings Robotest