DocsUI: Fix assertWindowTitle() for pre-material3

In pre-material3 when the device has `show_search_bar` the title isn't
displayed, instead the search bar takes its place.  Fix by searching in
the `header_title` where the location name is part of the text. E.g. for
"Recent" it says "Recent files".

Flag: EXEMPT test fix
Bug: 429356725
Test: atest DocumentsUIGoogleTests:com.android.documentsui.SidebarUiTest
Change-Id: Ieb0b77d6d4afec71b51b59711edecd3231ee2312