audio: Add PLATFORM_PC in AudioSystem

Add PLATFORM_PC in AudioSystem. And change getPlatformType to return
PLATFORM_PC when the device defines FEATURE_PC.

Bug: 414709012
Test: Together with the next CL, AL device is not unmuted on volume
down.
Flag: android.media.audio.enable_platform_pc_type

Cq-Depend: CL:33868354
Change-Id: I7ad68eb7f66c79adf3c7e9ac7a83401b653f00f7