Floss: Audit callbacks.

This CL ensures that duplicate callbacks aren't registered with
the same object path and returns the previously registed callback
ID if the same object path is used.

It also ensures that all add_callback DBUS functions also have
a corresponding remove_callback and standardizes the return type.

Bug: 287248056
Tag: #floss
Test: Unit tests and emerge-brya floss
Change-Id: I6c852f5b50ce13024e6c8f5a08238bd21d6d77b7