[DocsUI, Search]: Create search tasks once.

Creates tasks that load matches from a root once. Then for each cursor
that is completed after the deadline, forces the content refresh.

Bug: 436723718
Test: DocumentsUIUnitTests
Flag: com.android.documentsui.flags.use_search_v2_read_only

Change-Id: I8c0dd0691b9b78c537d2b300a821f4aee850ce95