Use view order when iterating icons in a slot

Before, slots handed out holders in the order they were added, meaning
you may try to insert the N+1th icon before the Nth

Change-Id: If993cf7233989ff22633f9b026aa3627bbc72d6b
Fixes: 69527817
Test: atest packages/SystemUI/tests/src/com/android/systemui/statusbar/StatusBarIconListTest.java