Update - there is an Android app SyncMyTracks.
Tested the free version - export from Strava, import to PolarFlow - yes, it works!
Free version takes just the last activity, but the apps costs 2,90 Euros - and it should really sync your data automatically, or you can select manually which activities to use.
Are there any users, who bought and use the app? It would be worth the money, if everything works as they promise!
Attention when using - I linked my PolarFlow with Strava and TrainingPeaks; after the import with SyncMyTracks sent Polar the “new” activity to both and I had to delete manually… so you should (temporary) break the links in Polar when importing.