Get current tracked BSSID from Layer2Info on starting provisioning.

In some circumstances where WiFi ScanResult is never used, the
current tracked BSSID will be null, it leads that there is no NUD
probe triggered evne if the L2 roaming does happen. Instead, fetch
the BSSID from Layer2Info class instead, which also includes BSSID
member and that comes from the current tracked BSSID in wifi.

Bug: 185202634
Test: atest NetworkStackIntegrationTests
Change-Id: I80b14fb4bd9c92a2ca4d79d5b9241f912df6e81b