Add bssid field in the ScanResultInfo class.

This change adds a new filed: bssid in the ScanResultInfo class
which are encapsulated by WiFi when connecting to AP successfully,
using current connected AP's bssid as a hint to determine whether
L2 roaming happened or not. Also adding @NonNull annotation for
fields, methods and constructor params.

Bug: 131797393
Test: atest NetworkStackIntegrationTests NetworkStackTests
Test: atest FrameworksNetTests
Change-Id: Ia11f1954124f45f9586495f6be7fb7fb3430ae97