assert plat neverallows on nonplat seapp_contexts

With the plat/nonplat policy split, nonplat_seapp_contexts should still
be checked against the plat_seapp_contexts_neverallows during build
time to ensure no violations occur.

Test: stock aosp_marlin builds.
Test: name=foo.bar seinfo=default fails (as expected) in nonplat policy
Test: name=foo.bar seinfo="" fails (as expected) in nonplat policy
Bug: 36002816
Change-Id: I95b2c695b23e2bdf420575d631e85391e93fc869