[DocsUI, Search]: Adds tests for SearchV2.

This CL creates 2 private method that hold test body and calls them from
4 different tests with the mix of search_v2_read_only disabled and
enabled.

Bug: 380988442
Test: m DocumentsUIGoogle
Flag: com.android.documentsui.flags.use_search_v2_read_only
Change-Id: Id5f3f49f44bd9c06d122e6a68065a1b04cc57a17