ProviderTestRule option to use non-mocked ContentResolver

### Description
For research and development, I want to write tests that explore the behavior of content-change notifications my ContentProvider sends via ContentResolver.notifyChange(). This is not possible with ProviderTestRule, as MockContentResolver has empty implementation for notifyChange().

### Request
Please extend ProviderTestRule with an option to use a real ContentResolver in place of MockContentResolver.

### Background
My ContentProvider is providing a client-side helper API for accessing content, and this implementation relies in part on content-change notifications from my ContentProvider. I need to be able to test those interactions in a testing context.

Start by reading ProviderTestRule and MockContentResolver, then trace how the rule creates and exposes its resolver. Done means an opt-in path can use a real ContentResolver for content-change notification tests while the existing mocked behavior remains available by default.

Written by the indexing model from the issue text.