Bug 869296 - Do 'Last Number Redial' when we receive ATD>n, r=gyeh
Bug 869296 - New mechanism to deal with AT commands 'AT+BLDN' and 'ATD>n', r=gyeh

According to HFP spec, when we receive AT command 'AT+BLDN', we shall respond
with:

  1. 'OK' if the call history is not empty. An outgoing call would be made
  2. 'ERROR' if the call history is empty. No call would be made.

However, with current implementation, we are unable to know whether the call
history is empty or not in Gecko. Therefore I introduce a solution, using a task
and a flag to decide if a call is made after AT+BLDN is received, which may not
be perfect but reliable, easy to understand and easy to implement.