[DocsUI, Search]: Add extras about slow search.

Collects information about cursors being ready. Records it in the extras
of the search cursor. Adds the test. Fixes RootCursorWrapper to
correctly handle extras. Adds tests for RootCursorWrapper to confirm the
fix and core RootCursorWrapper properties.

Bug: 417818526
Test: m DocumentsUIGoogle
Flag: com.android.documentsui.flags.use_search_v2_read_only
Change-Id: If5296fed706ca06d95c74aabcd9f27400cdd125b