wifidisplay: Modify WifiDisplayController

Spawn RTSP server before P2P connection
--In order to be sanguine that the RTSP server has been spawned
  on source, invoke listen() before P2P connection.
--Add a debug property to enable verbose logging in WifiDisplayController

Block teardown to avoid severing P2P group
--Since WFD dispose is asysnchronous, framework proceeds with teardown
  and dissolves P2P group before WFD stack is able to complete message
  exchange for teardown.
--Prevent framework from doing so and allow it to proceed only once
  WFD stack indicates it's dependence on network is over.

Add null check guards in WifiDisplayController
--The WifiP2PGroup delivered in the onGroupInfoAvailable() callback
  is not guaranteed to be non-null and hence needs to be validated.
--The same applies to the WFDDevice that is retrieved using
  info.getOwner().

Change-Id: Iee0a7f04f05beb402799117bed5a67e72f57dc9e