Do not do anything special to system bars when dispatching insets

The new insets system will not report insets above the window, and the
insets of the window itself. Besides, the system UI have the flexibility
to deal with other types of insets if they want to. The logic of
excluding the system bars for system bar windows is no longer needed.

Test: InsetsPolicyTest
Bug: 243758934
Change-Id: Iebf510b6863c642859d3eb91f3255c67993a3e7d