Allow overriding disabled changes in final release.

Since disabled changes are disabled by default, our policy should allow
opting in to them in the final release.
Add toString to OverrideAllowedState for easier test debugging.

Test: atest com.android.server.compat.OverrideValidatorImplTest
Bug: 144552011
Change-Id: Iff01ee44d30d3e5703a980cddaf1b38435756c3c