Fix ScreenDecorWindowTests on devices without nav bar

Navigation bar and screen decor windows would comsume the restricted
frame. By the current logic, only when the device has navigation bar,
app windows with SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION can extend to
the unrestricted frame. So, if the device doesn't have navigation bar,
app windows can never extend to the unrestricted frame (the insets
won't be affected by screen decor windows).

This CL removes the check about the existence of navigation bar.

Bug: 119084635
Test: atest ScreenDecorWindowTests com.android.server.policy
Change-Id: Ia7de52beb5b639c0c6a246bb1dfa16518c7509b1