[MaterialDatePicker] Unable to select date with espresso or/and reflection

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**Description:**
I can't select a date with espresso or/and reflection:
1. Espresso
I'm trying to do something [similar](https://github.com/material-components/material-components-android/blob/cfc00aa5a361f3c1c8a784fa597f03266af22e6e/tests/javatests/com/google/android/material/datepicker/MaterialDatePickerTestUtils.java#L172), but Month is final and package-private...I can select the day, but not the year or month.
2. Reflection
Doesn't work, I get error NPE: null receiver
```
onView(
ViewMatchers.isRoot()
)
.perform(customViewAction { rootView ->
val context = rootView.context
val activity = if (context is ContextThemeWrapper) {
context.baseContext as AppCompatActivity
} else {
context as AppCompatActivity
}
val fragment = activity.supportFragmentManager.findFragmentByTag("myTag")
val field = MaterialDatePicker::class.java.getDeclaredField("dateSelector")
field.isAccessible = true
val value = field.get(fragment)
...
})
```

**Expected behavior:** Add the ability to select a date in ui tests or add examples of how to do this, if it is possible right now with version 1.4.0.

**Android API version:** Any

**Material Library version:** 1.4.0

**Device:** Any

Start with tests/javatests/com/google/android/material/datepicker/MaterialDatePickerTestUtils.java, especially the linked selection helpers, and compare them with the MaterialDatePicker APIs available in version 1.4.0. Done means providing a reliable way to select the month and year in UI tests, either through supported test interaction or a documented working example.

Written by the indexing model from the issue text.