rilmodem: Fix GPRS attach/detach logic ( LP: 1234434 ).

When the primary context was shutdown, the active_ctx_cid was
being reset before rilmodem notified the core that the context
had been deactified.  This meant an invalid (-1) cid was passed
to the core, and thus the context wasn't being cleaned up properly,
which in turn could cause the attach code to quit responding to
subsequent attaches ( either from DBus requests, or DATA
registration changes.
rilmodem/gprs-context: re-factored code to make all context ids gints.

The rilmodem gprs-context code internally stored context ids
as guints, and used -1 to indicate IDLE/disconnected state.
As rild and the core ofono code only used gints, this led
to casting issues, and also was confused the code, as the
gprs_context_data struct also includes a status field which
more clearly be used to check state.
Merge pull request #8 from tonyespy/gprs-lp1234434

Gprs lp1234434