WifiDisplayController: handle preexisting p2p connection status

Handle the status that there is an existing p2p-group already formed
when starting to connect to wifi display.

There are two cases:
- if the desired device is the same with the already connected device,
  start to listen to the remote display.
- if the desired device is different, disconnect the old one and reconnect
  the new one.

CRs-Fixed: 2293770
Change-Id: Icacc59d38fb81d6a6f812043c2798760f82bc606