[DocsUI, SearchUI]: Add Search CUJ test.

Adds a test that checks that the search icon is visible in roots that
allow searching, and hidden otherwise.

Bug: 435522777
Flag: com.android.documentsui.flags.use_search_v2_read_only
Test: atest DocumentsUIGoogleTests
Change-Id: Icd5ac79b3efaf9cacd9013bb3f8d87641c56428c