Prevent causing insets if the system bar is shown forcibly

Forcibly-shown system bars are usually transient. Prevent them from
causing insets can reduce the UI shakiness, and can be compatible with
the behavior in the legacy insets mode.

Fix: 150582388
Test: atest InsetsSourceProviderTest InsetsStateControllerTest
            InsetsPolicyTest
Change-Id: I3c0fa4fb7555b2f63e1c4849db7b169489ab64e4