Floss: Refactor btmanagerd bin and lib

In Rust, if a crate is both a bin and lib, only lib will get custom
linker flags applied. In our case, we need to apply -lc++ to link with
libc++ and not libstdc++, and the current code structure results in
btmanagerd linking to both c++ libraries. To fix this, we move as much
code as possible to the lib so that all will be linked with libc++, and
main.rs should only contain glue code that uses all the components from
the lib.

Bug: 224606285
Tag: #floss
Test: Build Floss on Chrome OS

Change-Id: Id40ca1a7c42159d4df72c0734060750bbce18e03