feat($time): create time service

Add simple $time service to allow easier mocking dates in applications.

Closes #10402
The options we have here:

```js
// 1
this.now = $window.Date.now;

// 2
this.now = function() {
  return new Date();
};
```
1. Was requested in #10402 and is implemented in PR
2. Is what what you @caitp call object? I've build something like this in my project, and it worked nice.