[DatePicker] Make datepicker fullscreen

[manabu-nakamura]

closes #3956

[raajkumars]

@manabu-nakamura Thanks for the PR. But can you explain why the date picker must be made to occupy fullscreen always? For ex: if the date picker is shown on a large screen tablet, then we would not want it to occupy the entire screen.

[manabu-nakamura]

(before) The fullscreen datepicker is not fullscreen (edge-to-edge) in 2 & 3-button navigation mode. (after)