[MaterialDatePicker] Relax Visibility of Date Selection Components

Nobody has claimed this yet.

### Problem

Some of the `MaterialDatePicker` pre-built components have a restricted library scope; `@RestrictTo(Scope.LIBRARY_GROUP)`

This means that consumer apps cannot extends those classes, in order to implement more custom behavior, that is built on top of their base behavior.

Examples:
- [RangeDateSelector](https://github.com/material-components/material-components-android/blob/master/lib/java/com/google/android/material/datepicker/RangeDateSelector.java)
- [SingleDateSelector](https://github.com/material-components/material-components-android/blob/master/lib/java/com/google/android/material/datepicker/SingleDateSelector.java)

### Solution

Removing these Library Scope Restrictions would be much appreciated, so that we can extend custom behavior on top the existing components, without having to duplicate functionaility.

For example, if one wanted to create a month selector that would select a month in a date range of [1st of month - last day of month].

One could extend the `RangeDateSelector` implementation and slightly modify some of the behavior to accomplish such a case, without having to rewrite how `RangeDateSelector` works.


Start with lib/java/com/google/android/material/datepicker/RangeDateSelector.java and SingleDateSelector.java, linked in the issue, and inspect their @RestrictTo(Scope.LIBRARY_GROUP) annotations and related date-picker API usage. Done means consumer applications can extend these components without duplicating their existing behavior, while the project still builds successfully.

Written by the indexing model from the issue text.