aptrepo: make owners of incoming dir configurable

For different uses (apt.wm vs. releases.wm) we have
different users/groups owning the incoming directory.

Now that we use the same module for them we need to make
it configurable too, for uploads to work like before.

This is not changing the actual owner/group from how it was
before, it's restoring carbon and bromine as they were.

Bug:T132757
Change-Id: I62ecadc06c98b0a4e9e8b755a930e6c5cb38a206