[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.

Inizia da tests/javatests/com/google/android/material/datepicker/MaterialDatePickerTestUtils.java e confronta le relative interazioni di test con l’API RecyclerViewActions a cui si fa riferimento nell’issue. Definisci le azioni pubbliche di Espresso necessarie per selezionare una data senza esporre tipi privati, quindi verifica che gli utenti possano usarle per testare la selezione della data di MaterialDatePicker.

Scritto dal modello di indicizzazione a partire dal testo della issue.