Make DocumentsContract methods more general.

Accepting only ContentResolver arguments was quite limiting, so use
the newly created super-interface ContentInterface, which lets
callers use a ContentResolver, and ContentProviderClient, or even a
specific ContentProvider.

This is a safe API change, since we're accepting a more-general
argument, and existing API users can continue passing ContentResolver
to these methods.

Bug: 117635768
Test: atest DocumentsUITests
Test: atest android.appsecurity.cts.DocumentsTest
Change-Id: I109af667d9bdabe4bf78fb35fa61dee6c429ed47