[DocumentsUI]: Simplify tests.

Both the testFilesListed and testFilesList_LiveUpdates are flaky. This
CL simplifies the tests a bit. While this may not address the main
issue, it is an attempt to see if file name length, or duplication of
 tests plays a role in how flaky this test is.

Bug: 424299272
Test: atest DocumentsUIGoogleTests
Flag: com.android.documentsui.flags.use_material3
Change-Id: I31add08aa3b6da777d87319afff245f7099eb02c