Add AUDIO_FORMAT_MAT to the list of formats in XSD V6

This format is known to the framework since Android Q, and its lack
from the XSD was an issue, which only got fixed in
Audio HAL V7.

Bug: 176536594
Test: atest VtsHalAudioV6_0Target
Change-Id: I6c312a72e50e6547bd7f206d18ba7548b7bf8556