[MaterialDatePicker] For the dateRangePicker Do not nullify the end date when start date is re-selected

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**Is your feature request related to a problem? Please describe.**
The problem is, there may be a use case where the user needs to just update the start date and the date range could be very large. So having to also select the end date again can be frustrating to the user.

**Describe the solution you'd like**
Do not make the end date null upon selection of start date

**Describe alternatives you've considered**
Alternative could be to have two different date pickers for start and end dates which does not give the best user experience

**Additional context**
Add any other context or screenshots about the feature request here.

We also happily accept [pull requests](https://github.com/material-components/material-components-android/pulls).

Start by locating the MaterialDatePicker date-range selection handling and its existing tests. Check the behavior when a start date is re-selected after a range already has an end date; done means the end date remains selected without requiring the user to choose it again, with relevant tests passing.

Written by the indexing model from the issue text.