References to other resources are not supported by build-time PNG generation

重现：把minSdkVersion=21降级为19后出现这个问题。

说明及解决见此网页（要会“上网”）： https://stackoverflow.com/questions/46978566/references-to-other-resources-are-not-supported-by-build-time-png-generation

便于不方便“上网”的朋友，摘录如下：

image.png

image.png