The Firebase Android SDK for analytics provides a method named
Setting user property (FE): _id, 1234
I have spent a few hours on this and found the answer i was looking for more or less, hopefully this might save someone else some time.
The only way to filter by Firebase's User Id property is by creating an audience, there you will be able to pick the user id property that is supplied from the
setUserId(String id) method.
It is a downer though because you won't see any events for the user/user's you want from before the audience was created.