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上大神的回復，刪除.idea文件夾和所有的.iml文件眨层，然后重啟as锁右，重新build project，經(jīng)測試時有效果的很魂。
