DefaultNetworkEvent: fix two bugs with validation times

This patch fixes two bugs around the validation time calculation for
default network metrics:
  - for events representing the absence of a default network, the
    validation time was not correctly set to 0.
  - for events representing a default network, the validation time was
    always set to the duration time. This was overlooked by commit
    380a0638.

Also fix a minor printing padding issue in DefaultNetworkEvent#toString

Test: manually inspected the output of $ adb shell dumpsys connmetrics
      while enabling and disabling wifi.

Change-Id: I0eb4ccdf7a61d3097d0661104cb40d738e59772a