[MaterialDatePicker] Ability to reset selected day

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**Is your feature request related to a problem? Please describe.**

I am using MaterialDatePicker ('com.google.android.material:material:1.8.0') and I just realized that there is no way to reset the selected date to the MaterialDatePicker.Builder after being built of course. The issue comes after the following steps: the user opens the datepicker, selects a date, taps on one of the buttons or dismisses the dialog, reopens it again - the selected day stays where it left off. It would be nice to open again a new dialog with a non-selected date. However, after rotation the opened and selected datePicker, and then dismissing it and opening again - the date is unselected as it should be.

**Describe alternatives you've considered**
Before that I used CalendarView with public method setDate() which allowed me to handle this issue.
https://developer.android.com/reference/android/widget/CalendarView#setDate(long)

Start by reading MaterialDatePicker and MaterialDatePicker.Builder, focusing on how the selected day is retained when the dialog is reopened. Add a way to reset or clear that selection after building, and verify that reopening the picker shows no selected date while rotation state restoration continues to work.

Written by the indexing model from the issue text.