HDMI: Improve validation for <Set System Audio Mode>

Parameter should be exactly 1 byte long and can be either 00 or 01
(Off/On).
Message should be sent only by an audio system.
This complies with the HDMI Specifications.

Test: atest com.android.server.hdmi
Bug: 341680401
Change-Id: I354f6e81121a31e8dd784b5d2e1e35a0b8025d7d