Backup platform: Update OWNERS.

Strictly speaking other owners for SettingsProvider/test/...
should exist but there aren't any right now, so it's not
worth keeping a per-file rule since I don't think that
those can be combined with include.

Bug: 159055442
Test: Treehugger
Change-Id: I38c9c227e16c91f72ee0fc761670f82160e46ae2