Use createDocument instead of createFileInRoot

The createFileInRoot function in TestFilesRule is reserved for deferred
creation at a class level. The document creation that happens local to
this test should happen once the test has started AND only at the test
level. Prefer the createDocument function instead.

Fix: 408561790
Test: atest FilesActivityUiTest#testFilesList_LiveUpdate
Flag: EXEMPT test fix
Change-Id: I4a6990b8e83334799cfb6bb404d0c18b0999039f