[autopilot/sound] indicator-sound mock uses wrong signatures

[Jonas G. Drange]

Reproduce:
Run the test_sound tests

What happens:
The slider is 0.0, but the mock says it should be 0.4

What should happen:
The slider should indicate it's mocked value, 0.4

There are many reasons for why this is [1]:
* DescribeAll is mocked with in_signature 's' which should be ''
* DescribeAll is mocked with out_signature 'a{s(bsav)}' which should be 'a{s(bgav)}'
* Describe is mocked with out_signature '(bsav)' which should be '(bgav)'
* All structs are returned as dbus.Arrays.

[1] https://wiki.gnome.org/Projects/GLib/GApplication/DBusAPI