Removing passing defaults to ThemeSettings.

The settings API was simplified and now writing requires all fields, to be passed, meaning no sparse input and fallback to defaults.

Reading also will not fallback to defaults and will fail in case of bad formed json settings or else.

Bug: 333694176
Test: atest FrameworksServicesTests_theme
Flag: android.server.enable_theme_service
Change-Id: I0ae920f092e6e052e811db9f65915dc719245bb9