Convert some async to sync in btmanagerd

In preparation to use dbus_projection with btmanagerd, some interfaces
that interact with D-Bus needs to be sync instead of async due to the
nature of the floss API design where all methods should be sync (return
immediately) and async events are via callbacks instead:

* StateMachineProxy needs to be sync since this interfaces directly with
  D-Bus handlers.
* DbusCallbackUtil needs to be sync, but this is only transitional state
  because DbusCallbackUtil will be removed in the next patch when we
  integrate with dbus_projection.

All other internal code can stay using async interfaces.

This patch also fixes some compiler warnings.

Bug: 189501475
Tag: #floss
Test: Build floss on Linux

Change-Id: Id08792b2348944bd57b9ff7fbe97c43a2cb8ae8b