E911 call fix in ECM

Based on the VZW requirement, phone should be still in ECM mode in 2nd emergency call.
but in the current phone call, if a 2nd emergency call is originated, ECM mode will exit.

For fixing this problem, the coding design is as below:
1. In framework, canceling the first ECM timer immediately upon the origination of the
   2nd E911 call, and restarting a new timer when the 2nd E911 ends.
2. Framework needs to syncronize the timer with phone app by sending notification to phone app to
   inform timer is canceled or re-started, since phone app needs to show how much ECM time left
   on the status bar.
3. In phone app's emergency callback mode service, the timer in this service will be canceled
   when it receives the timer cancel notification from framework; the timer will be restarted
   once it receives timer restart notification from framework.