Floss: Provide dbus_generated! macro

The dbus_generated! macro is a convenient tool to mark a function that
the implementation does not matter since it will be replaced by
generated code. This frees developers from having to write placeholder
return data to be used.

Bug: 200732080
Tag: #floss
Test: Build Floss on Linux

Change-Id: I9ab147d611c8e541980ce4ea953c2f274909b9c4