Add MultiConfig for fallback logic

This change adds MultiConfig and HashConfig classes,
but does not actually use them anywhere. In a future
change, we can convert DefaultSettings.php into
a HashConfig instance and use MultiConfig as the
'main' config instance.

Bug: 69418
Change-Id: I0ef2fbb86d5c27602d70240219ee08be31e2d09b
Merge "Add MultiConfig for fallback logic"