Floss: Support callback objects in D-Bus client interface generator

With the convention that Box<dyn> parameter to RPC interface being an
exportable object via rpc, this adds support to dbus_projection macro
that adds the conversion of such parameter to D-Bus object path.

Bug: 200732080
Tag: #floss
Test: Build floss on Linux and test btclient manually

Change-Id: I103c73db9fd04c073ea69ef7377b26c5b9c4d4b1