When I bought my Xperia Pro-1, I hacked together a custom case for it that has a 67-72mm step ring grafted over a circular hole where the cameras and flash are. Normally I just have a clear glass ("UV protector") filter there.[1] The result looks a lot like Xiaomi's phone, but I can replace the glass if it gets damaged. Not sure if that's an option for their phone.

There are some side-effects. Sometimes there will be flicker from the flash (or maybe the active focus sensor?) that's reflected in the glass. Because phone cameras have such tiny apertures, dust and debris is more likely to be visible on the filter than on a full-size camera. I'm sure Leica can do a better job than me at minimizing those issues, but I wouldn't be sure they're non-existent.

[1] But of course it also works for circular polarizers, IR-bandpass filters, or anything else that comes in 72mm camera filter form.