Audio effect XSD: Give effect proxy its own tag

Effect and effect proxy do not have the same xml structure.
As they had the same xml tag, it was impossible to discriminate them in
the xsd. Leading to xsd validating incorrect xml.

This patch changes the xsd to force proxy effect to have the type
<effectProxy> instead of <element>.

With this patch, effects have to be explicit about being a proxy and
they are forced to have a hw *and* a sw library.

Bug: 37492580
Test: validated xsd against converted effects.conf
Test: vendor/box/user/mnaganov/audio-effects-conf/test.sh
Change-Id: Idf8a1040feccbfd6a346ab9bc892f3b7e87df530
Signed-off-by: Kevin Rocard <krocard@google.com>