[MaterialDatePicker] Create Espresso Actions

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**Is your feature request related to a problem? Please describe.**
It is very hard to write tests that need to interact with the MaterialDatePicker. This is made somewhat easier by following the code in your own [TestUtils Class](https://github.com/material-components/material-components-android/blob/master/tests/javatests/com/google/android/material/datepicker/MaterialDatePickerTestUtils.java) but due to private types like `Month` it's not possible to fully use this code.

**Describe the solution you'd like**
Using your test utils as a base, expose Espresso View Actions like was done for [RecyclerView](https://developer.android.com/reference/androidx/test/espresso/contrib/RecyclerViewActions). This will make testing these bits of UI much more simple.

**Describe alternatives you've considered**
It's possible to get things working, sort of, in an incomplete way, by using the `MaterialDatePickerTestUtils` as a sample, but it's a huge amount of work for the net gain of people able to write a test that selects a date.

Start with tests/javatests/com/google/android/material/datepicker/MaterialDatePickerTestUtils.java and compare its test interactions with the RecyclerViewActions API referenced in the issue. Define the public Espresso actions needed to select a date without exposing private types, then verify that consumers can use them to test MaterialDatePicker date selection.

Written by the indexing model from the issue text.