API: Work around wfMangleFlashPolicy()

The things wfMangleFlashPolicy() does to the output break things in the
API. For JSON we can work around it, while for PHP we just have to error
out. XML isn't affected because <> are escaped anyway (unless something
somehow uses 'cross-domain-policy' as a tag name), and the rest are
going away soon so they're not worth the trouble.

Bug: 66776
Change-Id: Idc5f37bd778288a9cde572f081dc753d681ec354
(cherry picked from commit e868f703ad4afbdcf81e7e392a46f20e356c4331)