[DocsUI, Search]: Fix testSearchV2LastModifiedDropdownVisibility

The test fails in compact layouts as the directory drawer button gets
hidden by the close search button. We factor out the code from
the testSearchV2RootNameIsAdjusted test into closeSearch() function in
the SearchBot. Then use this method in both tests.

Bug: 428057041
Test: atest DocumentsUIGoogleTests
Flag: com.android.documentsui.flags.use_search_v2_read_only
Change-Id: If9f252fe81298292d78efa70fd40862a2d1b8afe