audio: Add enable_platform_pc_type flag

Add enable_platform_pc_type flag that will be used in AudioSystem.java,
in getPlatformType function. This is to return PLATFORM_PC when the
device has FEATURE_PC.

Bug: 414709012
Test: Together with ag/33305462. Mute, then volume down. Device is still
muted.
Flag: android.media.audio.enable_platform_pc_type

Change-Id: I7f87c3bd2847c6e2ce0148c89ec2f4aaaf4b69e2