Refactor wifi p2p's startDhcpServer function

Add getTetheredDhcpRanges() interface and call it before calling
mNwService.startTethering to update dhcp ranges.  This will allow
p2p apps to run well concurently with other tethering apps.

Manual import of AOSP change 81546 by jianzheng.zhou@freescale.com

Change-Id: Iebc62f95bdcedde80e2c1d3e9580d3f625c3b50b