Very few flows actually check the phase. Push the checks down to the leaf
flows so that we can remove the inherited code from ResourceFlow and replace
it with utility methods. In the process, document and test two places that
throw the exception but did not previously test it.
This introduces a temporary hack in BaseDomainCreateFlow that does something
specific for DomainApplicationCreateFlow. It will go away literally tomorrow
when I flatten that flow.
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MOE_MIGRATED_REVID=135480538