[DocsUI, Search]: Add Search CUJ test.

Adds a test that checks that an empty query or freshly opened search
view retains the current directory listing.

Bug: 435522130
Flag: com.android.documentsui.flags.use_search_v2_read_only
Test: atest DocumentsUIGoogleTests

Change-Id: Ib713ecb52d674dc6f3383610dddd53a2b2a0c69d