audio: Do not unmute on volume down for PC

For TV, it's desired to unmute after turning down the volume. But it's
not desired for PC. TV is using single volume. And there are also PCs
that use single volume. So we need to check the platform is not PC
before sending MSG_UNMUTE_STREAM_ON_SINGLE_VOL_DEVICE message.

Bug: 394481728
Test: Mute, then press volume down, make sure it's still muted.
Flag: EXEMPT bug fix
Change-Id: I03da5e2699d3b5d879747f71f8bdfffefec96855