[floss]: Add floss dbus clients base class

Add a base class for Floss dbus clients. This contains some common
functions and declarations required for the manager and adapter dbus
clients.

Most of the const definitions here will be moved into an exported
header from the Floss daemon in a later release. For now, they will
remain in these headers.

BUG=b:189499077
TEST=autoninja chrome on entire chain

Change-Id: I568877ff90f82f8c9b60a5b4491e4e21838b1126
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3027300


Commit-Queue: Abhishek Pandit-Subedi <abhishekpandit@chromium.org>
Reviewed-by: Chris Mumford <cmumford@google.com>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Sonny Sasaka <sonnysasaka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#906347}