[DocsUI M3] Stabilize the tab cycle UI test

The grey overlay on the root Coordinator sometimes interfere with the
tab cycle UI test, e.g. the first tab press will focus on the root
CoordinatorLayout instead of the nav root. This CL attempts to fix
this issue by explicitly focusing on the root CoordinatorLayout first
before the test.

Bug: 427784540
Test: DocumentsUIGoogleTests:com.android.documentsui.KeyboardNavigationUiTest#testKeyboard_tabCycleInRootsList
Flag: com.android.documentsui.flags.use_material3
Change-Id: I208b9902b8b7c39a74cbe091121e2cebd0d01ced