Photos on Mojave is broken, not Android. I'm running High Sierra and Mojave on separate partitions, and Photos can import from my OnePlus 3T just fine on H.S., but not on Mojave.
No need to make excuses for app developers; if they don't want to support the photo transfer protocol that everyone has been using for years, people can make up their own minds how many new red marks go in the Apple column of their platform evaluation matrix. I use W10, Ubuntu, MacOS, and a couple of odd platforms, and right now, Mojave is the only one that can't do it.
I'll do the ol' feedback thing and expect to see it fixed just in time for 10.15 to be announced next June.
Meanwhile, there are several inconvenient workarounds based on copying the photo files to the Mac using, for example, Android File Transfer, which still works, and then importing them into Photos. Works, but is unpleasant.