Add dedicated mainline test mapping group for DocumentsUI

These presubmits are a separate suite configured to run these tests on
prior versions of android to check for backwards compatibility.

Following the instructions at go/mainline-test-mapping

Bug: 426030627
Test: atest --test-mapping packages/apps/DocumentsUI:documentsui-mainline-presubmit
Flag: TEST_ONLY
Change-Id: I63d7e8e8a523e5c7b9821069f8db52b5274a07f9