StatusBarDate: Add date to ClockViewModel

- Move longerDateFormat and shorterDateFormat from ShadeHeaderViewModel
  to ClockInteractor so that these flow can be use both in the clock and
  in the shade header.
- Add date text to ClockViewModel, which will be used to render the date
  text in status bar in the next CL.

Bug: 378545144
Flag: com.android.systemui.status_bar_date
Test: ClockViewModelTest
Change-Id: If5d3cc025b0ab4de36521c59d0ded7bc52af41d3