Wifi usability: Add IP reachability lost into Wifi unusable event

IP reachability is a strong indicator of Wi-Fi network quality. Extend
the WifiIsUnusableEvent list to include IP reachability lost.

Bug: 129282057
Test: frameworks/base/wifi/tests/runtests.sh

Change-Id: I0ff1075d619073d19fa365a1e6fc9894a109b8e9
Signed-off-by: Mingguang Xu <mingguangxu@google.com>