[MaterialDatePicker] Support different calendar input modes

Nobody has claimed this yet.

**Is your feature request related to a problem? Please describe.**
The current MaterialDatePicker forces you to select day, month, and year and is not customizable so that a user can only select day and month or month and year.

**Describe the solution you'd like**
Support different input modes where a user can select just day and month or month and year.

**Describe alternatives you've considered**
We needed to build our own custom Month/Year picker that uses DialogFragment. But we'd prefer not to build custom views for things like this.

The issue names MaterialDatePicker as the entry point; start by locating its current date-input behavior and how the picker is configured. Define how day/month and month/year modes should be selected and what each mode permits, then add coverage for both requested modes. Done means users can choose either mode without building a custom picker.

Written by the indexing model from the issue text.