Floss: Expose GATT API to DBus after interal API is ready

Make sure the API is ready in Floss layer and topshim api before
exposing the DBus API. Otherwise clients could poke the API too early
and cause crashes or unexpected behavior.

Bug: 300202055
Test: mma -j 32
Test: verify all interfaces are there via gdbus
Tag: #floss
Change-Id: I039c259c1c2cd32b4dbf781b36deff58cc7a9b4c