Fix default network transition event recording

When disconnecting from a default network X and falling back on another
connected network Y as the new default, ConnectivityService was
attempting to record this event as a X -> Y "atomic" transition.

In practice the default network connectivity is actually lost and
recovering default network takes some non-zero time.

This patch changes the event recording to always record disconnection as
X -> 0 events. At the same time, if there is a fallback network that is
elected as the new default ConnectivityService will also record a 0 -> Y
event.

This patch also improves pretty-printing of DefaultNetworkEvent.

Extract from $ adb shell dumpsys connectivity_metrics_logger --events
17:51:00.086: DefaultNetworkEvent(0 -> 100:CELLULAR)
17:51:25.232: DefaultNetworkEvent(100:IPv4 -> 101:WIFI) # wifi goes on
17:51:44.064: DefaultNetworkEvent(101:DUAL -> 0)        # wifi goes off
17:51:44.187: DefaultNetworkEvent(0 -> 100:CELLULAR)

Bug: 28204408
Change-Id: I63252633235bf6ba833b9ac431a80dda75a93e67