Integrate dbus_projection for btmanagerd

To reduce boilerplate codes, this migrates btmanagerd to using
dbus_projection framework:

* dbus_arg.rs: generated code necessary to use dbus_projection macros
* iface_bluetooth_manager.rs: API definitions
* bluetooth_manager.rs: D-Bus-agnostic implementation of the API, with
  logic copied almost unchanged from main.rs
* bluetooth_manager_dbus.rs: D-Bus projections
* main.rs: Moved D-Bus handlers to bluetooth_manager.rs
* lib.rs: Added iface_bluetooth_manager module so that the interface
  definition can be shared with btclient
* state_machine.rs: Migrated away from dbus_callback_util

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

Change-Id: I25b7e02243752fb0a1f2b75fb8384141e85bd91b