Effect VTS: Allow an effect proxy as pre/post processing

Effect proxy were previously mistakenly forbidden as post
or pre processing effects.

An effect being a proxy or not should not affect where
it can be use in the effect framework.

Bug: 111421676
Test: xmllint --noout --schema hardware/interfaces/audio/effect/2.0/xml/audio_effects_conf_V2_0.xsd $(find -name audio_effects.xml)
Change-Id: Iad1c1467226b86a2935a36dd90cf2e32f7f753b0
Signed-off-by: Kevin Rocard <krocard@google.com>