Replace lastVisitId with lastTimeStamp in PlacesInterestsUtils

Minor changes to placs

minor change
Replace lastVisitId with lastTimeStamp in HistoryReader
Randomly distribute visits through a day in test/helpers.js
Replace lastVisitId with lastTimeStamp in Controller and tests.
DayCountRanker computes an interest score as number of days when an interest visit happen
Remove enabled handling from Application
Rename onEnabled to restart in Controller and tests
Remove unused variable in DayBuffer
Ensure that Controller cleans storage onUninstalling