[FUI14] Get SSID from NetworkCapabilities

This allows subsequent patches to remove the need of exposing
SSID in NetworkState.

Test: atest NetworkStatsServiceTest
Bug: 174123988
Change-Id: Ib9ab07c9610954c9a5de035a3aefbf6d0bb0b53b

tests/net/java/android/net/NetworkTemplateTest.kt

@@ -67,6 +67,7 @@ class NetworkTemplateTest {
         val caps = NetworkCapabilities().apply {
             setCapability(NetworkCapabilities.NET_CAPABILITY_NOT_METERED, false)
             setCapability(NetworkCapabilities.NET_CAPABILITY_NOT_ROAMING, true)
+            setSSID(ssid)
         }
         return NetworkState(info, lp, caps, mock(Network::class.java), subscriberId, ssid)
     }