Finally, deleting .idea folder and all .iml files from the project, restarting Android Studio and rebuilding gradle did the trick. Autocomplete in XML support library is working again.
Probably checking out files from Version Control or copying all the source files in a new project would do the trick aswell.

stackoverflow上大神的回复，删除.idea文件夹和所有的.iml文件，然后重启as，重新build project，经测试时有效果的。