Prevent disabling checks via flags property

strict_updatability_linting currently only checks that updatability
checks aren't disabled via disabled_checks / warning_checks, you could
get around it using the `flags` property. Only allow controlling checks
via the *_checks properties.

Bug: 322802379
Test: m nothing --no-skip-soong-tests
Change-Id: Ia311622e1409f9cc201ab55f8749f2d13fccf551