Fix the declaration of setSystemGestureExclusionRects to match the PlatformMessageHandler interface (#12306)

Author: jason-simmons

shell/platform/android/io/flutter/plugin/platform/PlatformPlugin.java

@@ -94,7 +94,7 @@ public List<Rect> getSystemGestureExclusionRects() {
         }
 
         @Override
-        public void setSystemGestureExclusionRects(@NonNull ArrayList rects) {
+        public void setSystemGestureExclusionRects(@NonNull ArrayList<Rect> rects) {
             PlatformPlugin.this.setSystemGestureExclusionRects(rects);
         }
     };