Add top-padding to App Header when in full immersive

Adds padding equal to the status bar insets so that when the status bar
visibility changes (from the status bar window hidding in immersive and
being transiently shown on user clicks), the App Header is always shown
below the status bar.

Flag: com.android.window.flags.enable_fully_immersive_in_desktop
Bug: 369444147
Test: atest WMShellUnitTests
Test: manual - verify App Header is always under status bar in full
immersive mode

Change-Id: I91ea7c68a7d0faeaf5fb7b41e9b075bd52951ddd