Better docs for UriIdlingResource

### Description
I came across a usecase where I wanted to use UriIdlingResource in some WebView [tests](https://android-review.googlesource.com/c/platform/frameworks/support/+/1348702) which were flaky. The class description needs to be updated as it has a line that says it's still in beta which isn't true any more, I can see it has been part of multiple stable releases. I also think the class docs would benefit from having a sample code that demonstrates its typical use.

### AndroidX Test and Android OS Versions
the latest 3.3.0-rc01

### Link to a public git repo demonstrating the problem:
https://github.com/android/android-test/blob/master/espresso/idling_resource/net/java/androidx/test/espresso/idling/net/UriIdlingResource.java

Open espresso/idling_resource/net/java/androidx/test/espresso/idling/net/UriIdlingResource.java and review the class description. Remove the outdated beta wording and add a concise typical-use example, then check the generated documentation or relevant AndroidX test documentation to confirm both updates are readable and accurate.

Written by the indexing model from the issue text.