[DocsUI M3] Ignore navigation list divider in Talkback

The divider (spacer) item in the navigation item list should not
be announced by the Talkback.

Bug: 408888258
Test: m DocumentsUIGoogle && manual inspection
Flag: com.android.documentsui.flags.use_material3
Change-Id: I6454dc120cc905f8eb07bfc6a3cf62210ce83257