Add "VTS" mode to APM Serializer

This mode is used by VTS tests. In this mode Serializer
ignores all vendor extensions. It can't parse them
because VTS are always build from AOSP, w/o any vendor
additions.

Bug: 36733185
Test: atest HalAudioV6_0GeneratorTest HalAudioV7_0GeneratorTest
Change-Id: Id81c221446f3e118cbb97e7b2a73b2c1cee121b6