[flexiglass] Temporarily avoid relying on WindowInsets for ShadeHeader

This was introduced in ag/33290385, but due to a window layout param
we're currently getting 0 insets at the top. This will be fixed properly
in a follow-up CL.

Fix: 414424531
Bug: 414737230
Test: Manually by opening the shade on an unfolded foldable device where
 the problem appeared.
Flag: com.android.systemui.scene_container
Change-Id: I2886608b28a1822fb39ce1fe35feccc5943e80d1