Refactor `dyn I...` to be inside `Box<dyn I...>`

Many IBluetooth... interface accepts Box<dyn I...> as a callback.
This refactors the D-Bus projection utilities to put I... inside
Box<dyn I...> to accommodate passing the callback objects.

Additional refactor to accept String instead of &'static str as the
object path.

Bug: 188718349
Tag: #floss
Test: manual - Build floss on Linux

Change-Id: I40ec2aae30172704c5e4963173b752aea7b67a9f