Make the defaults property non-configurable

In order to support changing the global configuration per-module,
we can't make decisions on configuration until the configuration is
decided. The defaults mutator is one of the earliest mutators, and
it would be helpful to run it before deciding the configuration.

Bug: 361816274
Test: Presubmits
Change-Id: Iee9c603d7e2601919d636345dfdedae47448db38