[DocsUI, Search]: Fix testDeliversFastAndSlowResults

Test testDeliversFastAndSlowResults is taking longer than anticipated to
deliver loader results to the loader bindings. This CL increases the
interval between each root delivering results to 200ms, and increases
the first load time to 500ms.

Bug: 438347016
Flag: com.android.documentsui.flags.use_search_v2_read_only
Test: atest DocumentsUIUnitTests
Change-Id: Ifad25aec29be591740f3eadd2fc28eac42967aba