Support bit-perfect PCM playback.

This is part of USB audio improvement. Bit-perfect PCM playback
indicates that the PCM data will be sent as is down to the audio HAL.
When the track is bit-perfect, there will not be volume control
applied in the audio mixer. Only effects without processing or hw
accelerated effects will be allowed to attach to bit-perfect tracks.

Bug: 239435816
Test: manually
Test: atest audiopolicy_tests
Change-Id: I0bad4d7d78d4eaf741754d01bc5422ba15374782
diff --git a/services/audioflinger/AudioFlinger.h b/services/audioflinger/AudioFlinger.h
index a2cde70..333c57b 100644
--- a/services/audioflinger/AudioFlinger.h
+++ b/services/audioflinger/AudioFlinger.h
@@ -590,6 +590,7 @@
     class OffloadThread;
     class DuplicatingThread;
     class AsyncCallbackThread;
+    class BitPerfectThread;
     class Track;
     class RecordTrack;
     class EffectBase;