My project name contains ".", so the package name is not really valid since it was "P.C.P.P.C.P" (my project name is "P.C.P").
I edited it in the AndroidManifest.xml and it works pretty well- the pakcage name is now valid and the app runs perfectly on my phone.
- Note that I use Xamarin and not Android Studio.
So is it actually valid to change the package name in this way?