Floss: Add suspend API

This defines the suspend API at btadapterd level. The Suspend API is
supposed to be used in conjunction with another entity that actually
detects suspend/resume. In Chrome OS `btmanagerd` will communicate with
`powerd`, and in systemd-based Linux distros `btmanagerd` will
communicate with systemd using Inhibitor interface. In other Linux
systems suspend/resume script hooks can use this API as well.

Follow ups:
* Implementation of suspend() and resume()
* btclient command handler for debugging purposes.

Bug: 224606285
Tag: #floss
Test: Manual - Build floss with `build.py`

Change-Id: I5e9d3d37445aceded1224a7eaa6dfb564c25a3a6