Move Watchdog rollback logging to separate class

Decouple most of the watchdog rollback logging from Rollback
Package Health Observer. This allows for easier testing and
will facilitate future Watchdog logging requests from Staging
Manager.

Test: atest WatchdogRollbackLoggerTest
Test: atest StagedRollbackTest, check logs to ensure everything works
      as before
Bug: 138782888

Change-Id: I380c52322153873655ae07be11a3472926011345