Fix status bar content not being vertically centered on external display

On some devices a bottom aligned margin is defined in resources in order
to keep content aligned with a cutout.

This margin was being misapplied to external displays too, even when
they don't have a cutout.

Test: StatusBarContentInsetsProviderTest.kt
Flag: com.android.systemui.shared.status_bar_connected_displays
Fixes: 415946675
Change-Id: Ief6b46ee19981bc7ae38b2c073329c28f151afd6