Captive portal systel log improvements

This patch improves system logging around captive portal detection to
make inspection of bug reports sligthly easier:

- NetworkMonitor now logs by default CMD_CAPTIVE_PORTAL_RECHECK and
  CMD_CAPTIVE_PORTAL_APP_FINISHED. Other system logs are kept off with
  a new VDBG boolean contant,
- NetworkNotificationManager now prints the notification id at
  notification time. This allows to easily correlate show and clear.
- errors in NetworkNotificationManager are logged as Throwable instead
  of through their implicit toString() method.

Test: $ runtest frameworks-net
Bug: 32198726
Change-Id: Id74a9912880083d41e27753240e781595081ff7a