wifi: fix a flacky test result

The pno scan might be not triggered if device reconnect immediately
after disconnection. Adjust the test scquence to trigger screen off
first before disconnection. And remove pno scan trigger check to
avoid that pno isn't triggered unexpected like this reconnect case
since wpa_supplicant may also cause this reconnection happen.

Bug: 347149538
Test: android.net.wifi.mockwifi.cts.MockWifiTest#testMockPnoScanResultsOnMockWifi
Change-Id: I9ecb15a2141fdd5531a0879f17de0bc50f4bfcf4