Remove SystemUI exclusion for sandboxing

SystemUI was excluded from being returned an activity window's bounds
if the activity is eligible for size compat mode. SystemUI was
identified by the permissions check for INTERNAL_SYSTEM_WINDOW.
After confirming with SystemUI, this is no longer necessary.

Note that SystemUI has been returned DisplayArea bounds from the
display APIs for a few months now, with no ill effects.

Bug: 187491004
Test: atest com.android.server.wm.SizeCompatTests
Change-Id: Id2812cb389bb854ee02c11968fb960c9f044b175