floss: Add support for D-Bus method GetFlossApiVersion

This patch introduces a new D-BUs method for querying the Floss API
version. The major version of the Floss API is now defined as 1.
Future changes to Floss API will require updating the major version.

It also fixes a build warning message that appeared during unit test.

Bug: 300216447
Tag: #floss
Test: ./build.py --target test
Test: btclient floss show
Test: m -j
Change-Id: I038b2e0b961f0e3bdd571196c6f1b3431da3d14f