[MaterialDatePicker] Create Espresso Actions

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 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. 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.

Beginne mit tests/javatests/com/google/android/material/datepicker/MaterialDatePickerTestUtils.java und vergleiche dessen Testinteraktionen mit der im Issue referenzierten RecyclerViewActions API. Definiere die benötigten öffentlichen Espresso-Aktionen, um ein Datum auszuwählen, ohne private Typen offenzulegen, und überprüfe anschließend, dass Nutzer sie verwenden können, um die Datumsauswahl von MaterialDatePicker zu testen.