Data+MMS changes: Implementation for Legacy Api

- Add overloaded fucntion
  startUsingNetworkFeatureForSubscription
  stopUsingNetworkFeatureForSubscription
  This change is to allow the legacy MMS app
  to work for Data+MMS feature request.
  The purpose of this feature is to enable
  simultaneous PS ATTACH and PDP ACTIVE
- Fix duplicate requests. Add a check to
  prevent duplicate networkRequests being
  send to networkFactories

Conflicts:
services/core/java/com/android/server/ConnectivityService.java
Change-Id: I50bd6eea889a105a12d8b8d2beef6646675836fe