Add topshim for bluetooth interface

In order for the Rust front-end to use the bluetooth interface, add
a topshim to give Rust code access to C++.

Also includes some debug code in adapter/main.rs which will eventually
be replaced with a proper dbus frontend.

Bug: 183208689
Tag: #floss
Test: atest --host bluetooth_test_gd
BYPASS_LONG_LINES_REASON: Bluetooth likes 120 char lines

Change-Id: I58caec1ffadcc1d8d7e92f7c9669fa4f4d5bb91f