Fix child caching in opacity_layer (#17914)

Choose a child more likely to remain stable from frame to frame as the target to cache in the OpacityLayer.
https://dart.googlesource.com/external/github.com/flutter/engine/+/efe3f4569a6aa49935ffb9a688812f46a2971b89
diff --git a/commits.json b/commits.json
index 721d244..0836e42 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
-  "external/github.com/flutter/engine":"17737e6fd4ec28d83ecc1f159e50f9330f5befb1",
+  "external/github.com/flutter/engine":"efe3f4569a6aa49935ffb9a688812f46a2971b89",
   "external/github.com/flutter/flutter":"a1636b6fef2ebe7840ba6dc1b3f9d77095ef3b45",
   "sdk":"a2b5373eaac0fa36ac8fdda939b8fa26bd7d5633"
 }
\ No newline at end of file