Error when WITHOUT_CHECK_API=true in from-text stub build

Given that WITHOUT_CHECK_API=true is incompatible with from-text stub
build, this change makes the module raise an error when the two
conflicting configurations are set.

This change also modifies the checkapi error message, that
DISABLE_STUB_VALIDATION does not bypass checkapi.

Test: m nothing --no-skip-soong-tests && WITHOUT_CHECK_API=true m nothing
Bug: 322060779
Change-Id: I597309baa8fcfbefa05863c7b392bfb8b6774cea