Display-level relative display rotation should be 0

Because duh. DisplayContent is root, so it can't have a
relative rotation.

The problem is that RootWindowContainer ends up with the
default display's rotation because we store the "global"
configuration at that level. This is a legacy thing, so
until we clean that up, we have to just fix things at the
display level.

Bug: 249250882
Test: atest ChangeAppRotationTest, look at the trace and see that
      the recording display doesn't get rotated.
Change-Id: I76dc12be8ba934799dbc2ade3e00476f73bf3c71

services/core/java/com/android/server/wm/DisplayContent.java

@@ -6729,4 +6729,11 @@ class DisplayContent extends RootDisplayArea implements WindowManagerPolicy.Disp
     DisplayContent asDisplayContent() {
         return this;
     }
+
+    @Override
+    @Surface.Rotation
+    int getRelativeDisplayRotation() {
+        // Display is the root, so it's not rotated relative to anything.
+        return Surface.ROTATION_0;
+    }
 }