Fix NullPointerException for WifiDisplayController.

Set WifiDisplay to unavailable if p2p device is unavailable,
because the approach of WifiP2pWfdInfo.getWfdInfo() maybe return null,
so we need to do a non-null check to ensure the NPE issue will not
happen.

Bug: 297799091

Test: atest FrameworksWifiApiTests

Change-Id: I06e77719e646a28cc40e1e3d7c45b4f3c9472673
Signed-off-by: jiayongqiang <jiayongqiang@xiaomi.com>