[SB Refactor] Add null checks around network capabilities.

It's still not clear to me how the old code didn't NPE but ag/22820241
started causing some NPEs, because ConnectivityManager specifies that
the NetworkCapabilities from #onCapabilitiesChanged will be non-null
(and the old code didn't check nullability). But, let's fix the crashes
now anyway.

I don't believe we need to fix the new pipeline because I haven't seen
any crash clusters come through.

Fixes: 280169520
Test: atest WifiStatusTrackerTest
Change-Id: I1f100b9d799d220505ac4ebf133ee7aae551f833