Audio XSD: Allow vendor extension for audio devices

Without this patch vendors can not do any vendor extension extending
the audio enums as it would fails the XSD validation.

Long term, the config xml parsing should be moved in the HAL, until then
allow extension if they are prefixed with EX_.

Test: xmllint --noout --schema hardware/interfaces/audio/4.0/config/audio_policy_configuration.xsd  --xinclude out/target/product/*/vendor/etc/audio_policy_configuration.xml
Bug: 117967786
Change-Id: Ie44490824b5b50fdebb50a82299ea348d2a34b84
Signed-off-by: Kevin Rocard <krocard@google.com>