Add setShadowRadius for StubTransaction

So the test won't use invalid SurfaceControl.

Bug: 148807641
Test: ActivityStackTests#testSetAlwaysOnTop
Change-Id: I55ffc3acf6cb2c2472f6dce960edc0a8f527343e

services/tests/wmtests/src/com/android/server/wm/StubTransaction.java

@@ -260,4 +260,9 @@ public class StubTransaction extends SurfaceControl.Transaction {
     public SurfaceControl.Transaction unsetColor(SurfaceControl sc) {
         return this;
     }
+
+    @Override
+    public SurfaceControl.Transaction setShadowRadius(SurfaceControl sc, float shadowRadius) {
+        return this;
+    }
 }