Floss: Set up handling of D-Bus methods very early

Setting up the D-Bus method handler needs to be done before exporting
interfaces, so that when the InterfacesAdded signals are sent, we are
ready to handle D-Bus method calls immediately.

Bug: 232565047
Tag: #floss
Test: Manual - start btmanagerd, then btadapterd, check that btmanagerd
  does not panic

Change-Id: I2ea43f34e1934daf73df13fcd9ae59b184cccaf0