ENYO-442: Utilize method for temporarily suppressing events.

### Issue
We are currently using a pair of `silence` and `unsilence` calls to temporarily suppress events in situations where it is either necessary (to prevent undefined object errors) or helpful (to prevent unnecessary execution of change handlers). We've added a `muzzle` function that can be utilized now.

### Fix
`moon.DatePicker` has been updated to use the `muzzle function, while we've added the use of `muzzle` for `moon.TimePicker` to prevent unnecessary execution of change handlers.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>