This adds full integration tests for extension registration, ensuring
that ExtensionProcessor and ExtensionRegistry work together as expected.
The existing tests for ExtensionProcessor and ExtensionRegistry are
converted to "pure" unit tests.
This change is intended to protect against bugs while refactoring
ExtensionProcessor and ExtensionRegistry to better integrate with the
new SettingsBuilder class.
Bug: T297166
Change-Id: I2d9cc373b8a4aa014aef93b0a8e8d83758851783
