Given the Wild West nature of Android, there may be no other possible way to get that version of that system library to debug something.

I would have to hope that uploads are only triggered if the hash in the metadata wasn't found in their massive store of libraries.

You don't need "that version of the system library" to debug anything. If you write decent code that adheres to the published APIs, it works or it's Android's fault. Should it be Android's fault and you deploy a workaround, there's a pretty good chance your workaround will start breaking the moment Android fixes their bug. But this has nothing to do with that.

Let's face it. This is the freakin' Facebook app. It's not doing anything so incredibly revolutionary in the field of computing that requires it to be intimately involved in system libraries. It needs to display cat pictures and take in and emit text and make HTTP and HTTPs calls over the network, oh and monitor the user's every move, even while they sleep.

And what if that library that is uploaded is one that I wrote with no designed intent of public distribution?

It is wholesale going after every library on the system, AFAICT.