Sim writes

This PR adds supports for writing files in rilmodem. Tested on make image #250. To test this, a couple of things that can be done are:

* Write an array of strings in org.ofono.SimManager SubscriberNumbers property. This writes file EF_MSISDN, which is available in most SIMs.

* Set on/off Call Forwarding Unconditional. This writes file EF_CFIS.

* Use Message Waiting Indication. This writes EF_MWIS.

Depending on the operator, these file can be present in the SIM or not.