bootstat: Migrate to StatsLog for metrics

- All events are migrated to StatsLog atoms.
- Still keep the string based name for BootEvent logging as that makes it
  easier for debugging purpose.

Bug: 132691841
Test: check boot related stats are pushed after boot up
      $ adb shell cmd stats print-stats

Change-Id: I33313695a8a2d0a3c74f7705c0843c8c4bad2133