Wifi: Allow configuration of the P2P ifname

ICS used the same wifi.interface as the regular wifi service.
Let's use wifi.p2pinterface here, since they _can_ be different

Change-Id: Iab68b245b33377271354c849bbe33725a26193f1

Conflicts:
	wifi/java/android/net/wifi/p2p/WifiP2pService.java