[MaterialDatePicker] Added Support for a Spinner Mode

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### Context:
One of the modes supported from the older AppCompat Date Pickers was a Spinner mode.

This mode allowed users to use a set of dialers/spinners to select individual year/month/day directly.

Source: https://developer.android.com/guide/topics/ui/controls/pickers

It would be great to have `MaterialDatePicker` support such a mode, to align with use cases where it because difficult for a user to input an exact date, without having to scroll back in the Calendar View or Input a Text Entry Field.

### Example Use Case:
Is a user wants to select a particular month, from a given range of months:

In this example, if the user wants to select `March 2018`, having a spinner to do so, would lead to a faster experience than the Calendar View.

**Describe alternatives you've considered**
The alternative is to use the old `DatePickerDialog` which has support for the Spinner Mode.

**Additional context**
Source: https://developer.android.com/guide/topics/ui/controls/pickers

Start by comparing MaterialDatePicker with the older AppCompat DatePickerDialog and the Android picker guidance linked in the issue. The work is complete when MaterialDatePicker supports a spinner mode that lets users select the year, month, and day directly.

Written by the indexing model from the issue text.