Turn Clock into a class so its easier to inject dependencies

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>
Make cutoff_age_in_days for AppEvent and AppUsageEvent configurable

[#62823112] [#62482050]

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>