Update 19 March 2019
A new experience someone has just faced recently even though he/she did add a library module in app module, and include in Setting gradle as described below. One more thing worth trying is to make sure your app module and your library module have the same compileSdkVersion (which is in each its gradle)!
Please follow this link for more details.
Ref: Imported module in Android Studio can’t find imported class
Original answer
Sometimes you use import module function, then the module does appear in Project mode but not in Android mode
So the thing works for me is to go to Setting gradle, add my module manually, and sync a gradle again:
