IP connectivity metrics: add new APF counters.

This patch adds a few missing counters to APF events:
 - an actual lifetime duration to ApfProgramEvent.
 - counters for total number of updates to ApfStatistics.

ApfProgramEvents are now recorded at program removal in order to
populate the actual lifetime of the program. ApfProgramEvents whose
actual lifetime was less than 1 second are filtered out.

Finally, instance fields of ApfProgramEvent and ApfStats classes are
made mutable to allow for simple record-like creation. This was not
possible when these classes were tagged @SystemApi.

Test: - manually verified output of $ dumpsys connmetrics list
      - unit tests updated.
Bug: 34901696

Change-Id: I02694ebb9421ce1c2aa757fa6aa209d19a654dcd