WifiWatchdog changes for poor link detection

- use packet loss (wlutil pktcnt) instead of frame loss, retune all parameters
- use wpa_supplicant to get packet loss counters, instead of netd
- handle BSSID roaming in all situations
- improve flapping avoidance mechanism by setting different target RSSI
- handle high packet loss in high RSSI (never seen in real testing)
- add more comments on how to set all parameters

Signed-off-by yuhaozheng@google.com

Change-Id: I33429f063d8625a458be4791edd83a86d5a723df