Floss: Refactor DBus intf and only expose after interal API is ready

This does 2 things.

1. refactor the work of DBus interface registration to a separated
struct, i.e. InterfaceManager.

2. delay the DBus interface registration until the api is ready.

The reason why (2) is needed is because in many implementations, our
APIs assume the underlying libbluetooth API is ready when the DBus
method is called. If we register the API to DBus before it is ready,
there is a tiny window that clients could poke the API and cause crash
or get an unexpected result of it.

(1) is just to make implement (2) easier.

Bug: 299431339
Test: mma -j 32
Test: verify adapter/media/battery interfaces are there via gdbus
Test: perform mouse, kbd, headset CUJ on brya/skolas
Tag: #floss
Change-Id: If532032bf29b796e4fc46a68b90fa9ebc6a0756e