Well, I made the same steps like a few years before on E5. Back then it was successful, I need to type that amr-adition, got the information from unbuntu-forums... I don't know exactly, but it runs!
But now there is no success on Nexus 5... I flashed my phone again - no problem...
BUT: The sound quality from mp3-files is not very good. And I think, the fluendo decoder is the reason. I deleted it also from my desktop ubuntu machine (18.04) - and sound quality is much better!
So, if there is any idea to delete the fluendo-decoder and only run the ugly-plugins, so tell me please. And sorry, I'm only a normal user, I'm not very firm in Linux, but I'm very interested in it!
Thank you very much!