Handle call list in CallManager.

Update APIs to access foregroudCalls, backgroudCalls, and ringingCalls
     * 1. APIs to access list of calls
     * 2. APIs to check if any active call, which has connection other than
     * disconnected ones, pleaser refer to Call.isIdle()
     * 3. APIs to return first active call
     * 4. APIs to return the connections of first active call
     * 5. APIs to return other property of first active call

Change-Id: I195682dbce630eda80144e169bf84c17177f3368