Add support for Flexi Mapping

* Add two new interfaces getModemCapability and updateStackBinding
   - getModemCapability, for getting modem stack capability
   - updateStackBinding, for updating the stack and sub binding
* Modified sendOemRilRequestRaw utility method to receive variable
  number of payloads
* Introduce two new classes ModemStackController and
  ModemBindingPolicyHandler.
* ModemStackController:
    - Get Modem Stack capabilities on boot up with new RIL interface.
    - Notify Stack Capabilities are available to registrants.
    - Update Stack Binding based on preferred Stack combination received
      in the request for UpdateStackBinding. UpdateStackBinding involves:
        > Deactivate all active subs
        > Send Unbind/Bind stack as per request
        > Set pref NwMode on all subs after stack binding is updated.
    - Notify registrants that stack is ready after stack binding is
	  updated.
* ModemBindingPolicyHandler:
    - Based on the Network Mode selected for the subscription, determine
      the best stack which supports maximum RATs in the selected Network
	  Mode.
    - If Stack binding update is required, send request to
	  ModemStackController with preferred Stack Ids.
    - Based on result of UpdateStackBinding, send result to user.
* Listen for Stack Ready notification in SubscriptionManager to
  initiate activation of subs on all cards as appropriate.

CRs-Fixed: 668741

Change-Id: I0ae3bf76ba8e0965bb490f98e3c8060ca89fe192