Copied from libfeedback as of 2e081602f627505e566cc0bdb6cd96c7642d8b65
and adjusted for our mocking needs.
The mocked library will be LD_PRELOADED for the moment,
but further changes to the build should allow us to simply link
to it in the future.
We mock libfeedback and CallsCall to test the ringer for the following
scenarios:
- Call state changes
- User requested silencing of ringer
- Multiple calls
