DO NOT MERGE Captive portal systel log improvements

DO NOT MERGE 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

(cherry picked from commit 8b025bf108c729156b40159038befa0e6c5bebce)

Change-Id: I1eaab5ea702063dde3e23324d3a1b3dc172c5ac5