Make screenshot proxy always talk to USER_SYSTEM

Screenshot process is in the current user's profile, but the sysui
process is always user 0. We were talking to the current user sysui
instance which doesn't contain real data.

Bug: 399644508
Test: Manual test with external display, verify that proper focused
      display ID passed in
Flag: com.android.systemui.screenshot_multidisplay_focus_change
Change-Id: Ice5ae258fbf5b451d77a0b1e59f879ce70e51208