Floss: Add export_for_rpc to RPCProxy

This is a refactor that moves the logic of exporting a D-Bus object to
be inside the object itself that implements RPCProxy. This will make
DBus projection for btclient code easier.

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

Change-Id: I3f584b08ce86d77ca36642c4ef37b75a14fea649