Generalise call Provider abstraction and extract it to a library
This can then be shared with Wys. Reasoning:
- Code has been copy-and-pasted from Calls to Wys, resulting in fixing this bug twice: !81 (merged) wys!19 (merged)
- Other people (e.g. Plasma Mobile) want to use Wys with oFono
- The abstraction has got a little messy and needs dealing with; creating a public API would necessitate that
Related issues: #6 #92 #5 (closed)